Nanostructured Materials and Architectures for Advanced

Transcription

1 Review Infrared Photodetection Nanostructured Materials and Architectures for Advanced Infrared Photodetection Fuwei Zhuge, Zhi Zheng, Peng Luo, Liang Lv, Yu Huang, Huiqiao Li, and Tianyou Zhai* Infrared photodetectors are finding widespread applications in telecommunication, motion detection, chemical sensing, thermal imaging and bio-medical imaging, etc. The nanostructured materials and architectures are attracting extensive interests in photodetectors in view of the potential benefits from confined light-matter interaction, fast carrier dynamics and ultrahigh photoconductive gains. This review concentrates on the photodetection in the infrared spectrum and recent progresses in constructing advanced infrared photodetectors based on quantum wells, dots, and the rapidly evolving 1D and 2D materials are summarized. The recent achievements in exploring nanostructured plasmonic metamaterials for the intriguing subwavelength photon confinement and waveguides in devices are also surveyed considering their importance in device integration. An outlook of infrared photodetection is given in the end as a guideline for this vigorous field. 1. Introduction Photodetectors that convert light to electronic signals constitute the kernel of many devices regularly found in our daily lives, represented by the prevailing digital camera. Beyond the visible spectrum, infrared (IR) photodetectors are especially useful for night vision, motion detection, telecommunication, gas sensing, thermal imaging, bio-medical imaging and astronomy observatories. [1] This involves the detection of photons with broad-spanning wavelengths from 0.75 µm to >1000 µm, covering the spectrum of near IR (NIR, µm), shortwavelength IR (SWIR, µm), mid-wavelength IR (MWIR, 3 8 µm), long-wavelength IR (LWIR, 8 15 µm) and far IR (FIR, µm). Depending on the target applications, different spectrum bands are often used (Figure 1), and the practical requirements for infrared photodetectors also vary significantly in terms of their cut-off wavelength, detectivity, response speed and operation temperature. Though Si is dominating in almost every corner of existing semiconductor technologies, its indirect bandgap ( 1.1 ev) restricts Si based photodetectors only Dr. F. W. Zhuge, Z. Zheng, P. Luo, L. Lv, Y. Huang, Prof. H. Q. Li, Prof. T. Y. Zhai State Key Laboratory of Material Processing and Die & Mould Technology School of Materials Science and Engineering Huazhong University of Science and Technology Wuhan , P. R. China zhaity@hust.edu.cn The ORCID identification number(s) for the author(s) of this article can be found under DOI: /admt absorb photons with wavelengths less than 1.2 µm, being transparent to most of the infrared spectrum. [2,3] The exploration of alternative infrared sensitive materials thus constituted most of the early efforts in developing infrared photodetectors. [4,5] In this period, Hg 1-x Cd x Te (MCT) that has widely tunable bandgap from 0 to 1.5 ev covering the broadband infrared spectrum, has been extensively studied in IR photodetectors. [6 8] As the so far most successful infrared material, MCT based stateof-art infrared photodetectors still suffer issues in precise stoichiometry control of Hg 1-x Cd x Te and poor lattice compatibility with Si electronics, [7] both of which are however the prerequisites for chip integrated IR photodetector arrays. Meanwhile, the rapid evolving nanotechnology continuously renders researchers with unprecedented ability in engineering the optoelectronic properties of materials at the nanoscale. [1,9 15] In the specific field of photodetection, this has led to the blossom of high performance photodetectors based on nanostructured materials and architectures in the last decade. [9,16 18] For photodetection, an apparent advantage of nanostructured materials arises from their ultimately reduced sizes down to even atomic dimension. The reduced physical size directly leads to the ultrafast carrier transit time of ps in devices and greatly facilitates the construction of high speed photodetectors for optical communication and high frame cameras. [19 21] In the meantime, the synergetic manipulation of light-coupling, carrier dynamics and gain mechanisms in devices by arbitrarily constructed heterostructures enables a comprehensive optimization of infrared photodetectors. [1,17,22] The photodetectors assembled with hybrid PbS quantum dots and 2D graphene have reached the ultrahigh detectivity of over Jones with exceedingly high photocarrier gain (>10 8 ) in the SWIR band. [23] P-n or Schottky junctions integrated in InSbAs, [24] GaAsSb nanowires, [25] graphene and some hybrid materials have been recently developed for photodetection. [26 29] They have displayed greatly suppressed dark current in photodetectors and showed the great prospects in building room temperature photodetectors in the MWIR and LWIR band. With their unique optoelectronic properties, nanostructured materials are also adding additional functions to existing photodetectors, for instance, the color- and polarization-sensitivity. They are now feasibly built by utilizing the quantum effects and structural anisotropy in low dimension systems, such as quantum wells, dots, nanowires and 2D anisotropic materials. [30 36] From the perspective of light coupling to the tiny photodetectors beyond (1 of 26)

2 the diffraction limit, remarkable progresses have been also made in enhancing the light-matter interaction in the subwavelength volume, by using plasmonic nanostructures and metamaterials. [37 45] Upon integration in focal plane arrays (FPA), this improvement provides a solution toward high resolution infrared imaging sensors for thermal imaging. [46] These achievements from different aspects of a photodetector (detectivity, speed, operation temperature, etc.) clearly point out the potential of nanostructured materials and architectures in high performance infrared photodetection. In spite of the marvelous merits of nanostructured materials, a comprehensive leverage of photodetection performance in the infrared spectrum is still challenging, since it would rely on the synergetic integration of hybrid nanostructures form macro to nanoscale. In this review, we survey the current status and trends of infrared photodetectors based on various nanostructured materials, from quantum wells, dots to nanowires, nanotubes, 2D layered materials and their hybrid combinations. An overview of the infrared photodetection will be given in the beginning from the aspects of the basic principles and the materials choices at different IR bands. Then we concentrate on the novel infrared photodetectors enabled by different nanostructures and architectures. An outlook of future challenges and opportunities in the development of advanced IR photodetectors will be given in the end for a guideline. In sight of the widespread applications of infrared photodetectors and the intriguing optoelectronic properties of nanostructures, we hope the present review will help the researchers to gain a comprehensive understanding of this vigorous field and attract more research interests to join the challenge. 2. Principles and Materials for Infrared Photodetection 2.1. Principles and Figure of Merits of Photodetectors The photodetectors are usually built based on the photoelectric or photothermal effects in photo-absorbing materials. In the photoelectric effect, the absorbed photons are directly converted into electron-hole pairs and are detected as resistance or potential change in external circuit. [1] The complementary metal oxide semiconductor (CMOS) image sensors and high speed Si, GaAs photodetectors are constructed based on this effects. In contrast, the absorbed photons are transformed into lattice heat in the photothermal effect and are detected via sensing the local temperature change using resistive thermometers. [5,47] Bolometers and pyroelectric detectors that had been widely used in astronomy and thermal imaging devices belong to this type. For bolometric photodetectors, the readers are referred to the review by Richards and the book edited by Walcott et al. [48,49] Recently, there have been also growing interests in constructing graphene-based bolometers utilizing the weak electron-phonon coupling characteristic, [50,51] a review was recently given by Xu et al. on this emerging area. [52] In this review, we will only on the photoelectric type photodetectors, considering the wide application of nanostructured materials in this field and their obvious advantages in response speed, gain mechanism, feasible color and polarization sensitivity upon integration, etc. Fuwei Zhuge received his Ph.D. degree from Shanghai Institute of Ceramics, Chinese Academic of Sciences in Then he joined Osaka university and Kyushu university as postdoctoral researcher. He is now an associated professor in the school of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests have been focused on the controllable fabrication and application of various low dimensional nanostructured materials in photodetection, energy storage and conversion. Tianyou Zhai received his B.S. degree in chemistry from Zhengzhou University in 2003, and then received his Ph.D. degree in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) under the supervision of Prof. Jiannian Yao in Afterwards he joined in National Institute for Materials Science (NIMS) as a JSPS postdoctoral fellow of Prof. Yoshio Bando s group and then as an ICYS-MANA researcher within NIMS. Currently, he is a Chief Professor of School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST). His research interests include the controlled synthesis and exploration of fundamental physical properties of inorganic functional nanomaterials, as well as their promising applications in energy science, electronics and optoelectronics. The basic working principles of photoelectric type photodetectors and the essential figure of merits used to assess a photodetector are described in the following section The Working Principles of Photodetectors In photoelectric detectors, the resistance of a photosensitive material changes upon light irradiation due to the photon excited electron-hole pairs contributing to electrical conductance. Depending on the detailed configuration of photodetectors, photoconductive and photovoltaic effects can appear and dominate the photoresponse signal in devices. Understanding these effects is critical to distinguish the different behaviors of various photodetectors found in literature. [20,53,54] Photoconductive Effect. Photoconductive detectors are generally operated with an applied external bias voltage (2 of 26)

3 Figure 1. The typical photodetector applications at different detection spectral bands, from the ultraviolet (UV), visible (VIS) to the broad infrared, including NIR, SWIR, MWIR, LWIR and FIR&THz. As illustrated in Figure 2a, the electron-hole pairs generated from light absorption in this case are separated and swept to the opposite device terminals by the applied electrical field. The photocurrent is generated if the photocarriers are successfully collected to the contact electrodes before they recombine. Thus, the quantum efficiency (QE) of this carrier collection process is determined by the transit time of photocarriers through the device and the lifetime of photocarriers in semiconductor. QEs of more than unity (100%) are possible if the lifetime of photocarriers exceeds the transit time (in this case, the excess photocarriers tend to circulate in the conductive channel until they recombine). [54] Photovoltaic Effect. In contrast to photoconductive detectors, photovoltaic detectors utilize the built-in electrical field in p-n junctions or Schottky junctions to separate the photogenerated electron-hole pairs. The need of an external bias is thus omitted. As a result, photovoltaic detectors are capable to work with almost zero dark current. This characteristic is particularly useful in infrared photodetectors when integrated in FPAs, since it avoids the charge saturation in readout circuits by the notorious dark current in narrow bandgap materials. [7,46] Figure 2b illustrates the charge separation under the photovoltaic effects. Since the required built-in electrical fields generally exist in junctions between materials with work function difference, photovoltaic type photodetectors could be feasibly constructed from doping or simply hybrid materials combinations. [26,27,55 57] The practical photodetectors are constructed using either a planar or a vertical configuration, as illustrated in Figure 2c,d. The structure shown in Figure 2c corresponds to a planar photoconductive detector based on the common metal-semiconductormetal (MSM) configuration. [58] It is however noteworthy that the actual type of contacts may render the coexistence of photovoltaic and photoconductive effects in photodetection, since Schottky contacts are easily formed at metal-semiconductor junction. [20,53] The typical configuration of photovoltaic detector with a vertically stacked p-n junction is shown in Figure 2d. This configuration is not exclusive, since planar p-n junctions and Schottky junctions are also widely adopted in recent photodetectors. [59] It should be noted that in photovoltaic mode, the light absorption is preferred to locate within the junction area, since out of it the carrier separation efficiency becomes poor in the absence of built-in electric field Figure of Merits of Photodetectors Figure 2. The energy band diagram in a) photoconducive and b) photovoltaic photodetectors. Their typical configurations in devices with planar and vertical structure are shown in c) and d). The performance of a photodetector is examined by its ability to detect a target signal from background noises. In a photoelectric detector, this is determined by the light absorption efficiency, photon-to-electron efficiency, and the noise characteristics of the target, environment and photodetector itself. [5] Though different figure of merits are often emphasized for the photodetectors applied in different fields, i.e., speed for optic communication, detectivity for astronomy, the following parameters are universally used to describe the performance of a photodetector: Responsivity (R). The responsivity (R) of a photodetector is ratio of detected signal (I ph or V ph ) in a photodetector to the incident power of light P in to the active area of the photodetector, as R = (I ph or V ph )/P in. The responsivity is determined by the wavelength (λ) of target light, photo absorption efficiency (3 of 26)

4 (α), quantum efficiency of photon to electron excitation (η), and the photoconductive gain (g) of the detector: R = αηg*eλ/ hc, where h is the Plank constant, e is the electron charge, c is the light speed. The photoconductive gain is related to the ratio of the carrier lifetime and their transit time in devices, and is thus generally improved in nanodevices due to the reduction of transit distance. [60,61] In literatures, external quantum efficiency of a photodetector is usually used to describe the overall phototo-electron efficiency upon photodetection. It is defined as the ratio of electron fluence (electrons per second) in photocurrent to the flux of incident photons (photons per second). Bandwidth (Δf). The bandwidth of a photodetector describes its response speed to frequency modulated light signals. It could be extracted from the rise and fall time of photoresponse, as Δf = 1/τ. Alternatively, 3dB bandwidth Δf 3dB is often used, which is defined by the modulation frequency at which the response of detector drops to its half (3dB). The bandwidth of a photodetector is limited by the transit time and lifetime of photocarriers in the photoconductor. Upon integration, it is also limited by the RC time constant of readout circuit and the active layer itself. The transit time of carriers in the conduction channel is determined by the carrier mobility in semiconductor, the electrical field E and the channel length L in device: τ tran = L/µE. In contrast, the carrier lifetime τ rec in semiconductors are more complicatedly related with the presence of trap states, either at surface or in the bulk. In nano-scaled photodetectors, the transit time is greatly reduced to be as short as sub ps, whereas the carrier lifetime can vary largely due to the different traps states involved in devices. Hence, nanoscale photodetectors not only hold the promise in increasing the operation speed of photodetection, they also render higher photoconductive gains from the aspect of responsivity. Noise Equivalent Power (NEP). NEP is defined as the power of incident photons which generates noise equivalent response signals in the photodetector under the specific bandwidth of 1 Hz. It denotes the minimum light power detectable by a photo detector from the universally presenting noises, which includes the shot noises from both the background photon irradiation and dark current transport, and also the flicker noise from electronic device. The smaller the NEP, the higher detectivity of a photo detector can reach. In typical infrared sensitive materials, due to the narrow bandgap, the thermal generation of carriers tends to raise dark current in devices and cause significant noises at high temperatures. [62] Therefore, the noise issue is more critically emphasized in infrared photodetection compared to the case in UV-Vis detection. Regarding to the dominant noise from dark current, the dark current limited detectivity are often estimated for infrared photodetectors to depict their potential detectivity. However, at low frequencies, Flicker noise (1/f noise) often dominant the noise characteristic in devices, a direct noise measurement of in working photodetectors is thus necessary to avoid the overestimation of detectivity. [63 65] To realize the best detectivity, infrared detectors are often placed in cryogenic temperatures to suppress the related noises. For thermal imaging purposes, an identical parameter as NEP is the noise equivalent temperature difference (NETD). It is defined as the equivalent temperature difference that causes a signal-to-noise ratio of unity in photodetector, and is generally used to examine the ability of photodetector to distinguish subtle temperature difference in imaging target. Specific Detectivity (D*). The specific detectivity is a normalized measurement of the performance of a photodetector to its area A and bandwidth Δf. It is inversely proportional to the noise equivalent power, as D* = (AΔf) 1/2 /NEP. This parameter is universally used to compare different photodetectors in addition to the responsivity and speed, since the latter two can be significantly different if the photodetectors are differently configured. Considering that the noises in a photodetector is mostly dominated by the shot noises from dark current, the specific detectivity of a photodetector is often estimated from the dark current (I d ) and responsivity (R) measurement, D* = A 1/2 R/(2eI d ) 1/2. Depending on the target applications, different figure of merits may be preferred for a photodetector. For instance, in telecommunication, high bandwidth is the foremost prerequisite for data communication, while detectivity is less demanded due to guaranteed incident power from lasers. In contrary, for astronomy purposes where the targets hold relatively still, the detectivity is ultimately pursued to detect single photon incidence. The highly developed impurity control in semiconductor processing (Si, Ge, GaAs, etc.) in the last decade has greatly promoted the improvement of photodetectors bandwidth. As to the responsivity and detectivity, significantly progresses have also been made in the meantime in enhancing the photoconductive gains in photodetectors by introducing avalanche mechanisms, photogate effects. It is generally expected that the adoption of novel nanostructured materials and architecture designs would render a comprehensive optimization of photodetector. [11] 2.2. The Material Choices for Infrared Photodetection A variety of semiconductor materials have been implemented in infrared photodetectors at the different IR bands. The inpriori selection rule for materials in photodetection is by examining their absorption. In the infrared spectra, the absorptions in materials could be due to the electron transitions or phonon absorption. The electron transitions are further divided into interband and intraband transitions depending on whether the energy levels lie within the same energy band (conduction band or valence band). [47] In this section, we introduce the infrared materials by their underlying absorption principles and their detection bands. Interband absorption in materials are simply determined from their bandgap. For infrared detection, the materials should exhibit narrow bandgaps of at least less than 1.6 V. In these materials, photons are absorbed by the electron transition from an occupied energy level in valence band to an unoccupied energy level in the conduction band. The detection cutoff wavelength could thus be judged from their bandgaps. Figure 3 shows the bandgap of typical materials used in infrared photodetection. [5] As the most successful semiconductor materials, Si and GaAs has the bandgap only available for NIR photo detection. In the other bands of infrared spectra, they are replaced by materials like Ge, [66,67] PbS, [68,69] PbSe, [70,71] alloy compounds (InGaAs, InAsSb, HgCdTe, etc), [72 74] and recently graphene and black phosphorous (BP). [20,35,75 78] In the case of alloy compounds, the detection cutoff wavelength could be continuously tuned from far-ir to NIR by adjusting (4 of 26)

5 signals at the long wavelengths. [58] Other absorption mechanisms, such as surface plasmon resonance (SPR) induced absorption, have been also explored for infrared photodetection. [92 94] The photon absorption is related to the excitation of collective oscillation of free carriers. So far, the SPR absorption in metals is limited up to NIR spectra due to their high carrier concentration. Recently, semimetal and heavily doped semiconductors, e.g., TiS 2, heavily doped ITO, have been shown to exhibit SPR induced absorptions deep into the SWIR band depending on their doping concentration and structural geometry. [94] It is noticed that the unceasing developments of infrared photodetection technologies is only enabled by the continuous evolving of novel materials by the nanotechnology revolution in the last decade. Figure 3. The energy gap of typical infrared materials versus their lattice constant. Reproduced with permission. [5] Copyright 2010, Elsevier. the stoichiometry. In Figure 3, the tunable bandgap ranges of different materials combinations are indicated by the line connections. Herein, the binary materials with less lattice mismatches usually render more efficient alloying when combined into ternary materials, i.e., GaAs and AlAs, HgTe and CdTe. Alternatively, the interband absorption edge of materials could be also tuned the by quantum effect in nanostructured materials, e.g., by making them into quantum dots. [79] The released stress in nanomaterials further renders them excellent substrate compatibility up on integration. Intraband absorption occurs in materials by the electron transition between the energy levels within the same band (CB or VB). Thus, it enables the photon detection beyond the bandgap limit of materials. However, to enhance the intraband absorption, the materials are often heavily doped to populate sufficient carriers into the energy band involved in transition. [80] Intraband absorption constitutes the basis of photodetectors based on epitaxial quantum wells (QW) and dots (QD) of GaAs/ AlGaAs, [73,81] InAs/AlGaAs, [82,83] etc. [84] In these quantized systems, the electron energy levels in well or dot layer become discrete. As a result, QW and QD in devices often exhibit narrow band photoresponse. Moreover, due to the different degree of quantum confinements, the energy differences between adjacent levels are size dependent and could be tuned from mev to higher. These attractive features of QW and QD in devices had been utilized for the development of color sensitive infrared photodetectors, taking advantage of the feasibly tunable absorption wavelength in QW and QD by engineering their geometry parameters (width, size, barrier heights, etc.). [85 89] In addition to the interband and intraband absorption, the infrared light could also be absorbed by the optical phonons in materials. The light absorption in this case directly leads to the photothermal effects. Usually, this is not desired for photo electric detection, especially for the long wavelength photons in the FIR and THz. Recently, there are efforts in exploring III Nitrides for THz photodetectors considering their large optical phonon energy (corresponds to the frequency of 22 THz) that is transparent to THz photons. [22] However, the direct phonon absorption may also contribute infrared photodetection by the photo-thermoelectric effects, [90,91] though the signals tend to be less significant compared to the photoelectric 3. Nanostructured Infrared Sensitive Materials for Photodetectors The electronic and photonic confinements by the miniaturized physical sizes of nanomaterials endow various nanostructured materials with unique optoelectronic properties in photodetectors. Enhancing the photodetection performances becomes possible by simply tailoring the corresponding material properties. In this section, we survey the recent progresses in IR photodetectors based on the variety of nanostructured materials, categorized as (1) epitaxial quantum wells, dots and multilayers, (2) solution casted colloidal quantum dots, (3) 1D nanowires and nanotubes, (4) 2D materials, and (5) hybrids heterostructures. Table 1 listed a brief summary of the infrared photodetectors and their fundamental performances in terms of the responsivity, detectivity, response speed and important technique notes. Their structure related unique optoelectronic properties and functions in devices will be highlighted for a comprehensive vision of this rapid developing field Epitaxial Quantum Wells, Dots and Multilayer Films Quantum wells and dots based IR photodetectors are in the beginning proposed to replace HgCdTe photodetectors in the MWIR band considering their potential in high operation temperature and the prospects in high density chip integration. [72,102,103] In practical, quantum wells and dots are defined in sandwiched structures with a well or dot layer capped in barrier layers. From the technologies developed in quantum well lasers, epitaxial superlattices of GaAs/GaAlAs, [73] InAs/ GaAs, [104] Si/Ge, [105] etc., are widely used for the construction of quantum wells and dots based infrared photodetectors. Using molecular beam epitaxy (MBE) methods, the precise tailoring of the geometry parameters of quantum wells and dots (width, size, periods, etc.) have already become possible. Such advance in technology has greatly facilitated the accurate design of photo detectors, especially on its detection wavelength. Now, IR photodetectors based on epitaxial quantum wells and dots have covered the photodetection in the broadband IR spectra from THz to SWIR. Quantum wells and dots are routinely fabricated by sandwiching a well or dot layer in energy barriers through epitaxial (5 of 26)

6 Table 1. A list of the typical nanostructured materials and their performances in infrared photodetection. Category Detection bands Material Wavelength [µm] Epitaxial QWs, QDs, and multilayers Responsivity [A W 1 ] Detectivity [Jonse] Speed [s, Hz] Ref. Technical Notes SWIR to FIR InGaAs/GaAs QDs [73] Strain relaxed Narrow band InGaAs/InP QW [41] Plasmonic enhancement InGaAs/GaAs/AlGaAs QDs [87] GaAs/AlGaAs QCD [95] Photovoltaic mode GaSb/AlSb Barrier QCD [96] Electron barrier introduced Solution casted QDs SWIR to LWIR PbS QD Hz [56] PbSe QD khz [54] Field effect tuned HgTe QD KHz [32] HgSe QD [97] Plasmonic enhancement HgSe QD [36] 1D nanowires NIR to SWIR InP ms [98] Field effect depletion InAs NW [99] Schottky contact InAs NW ms [100] Negative photoresponse 2D materials NIR to FIR Graphene QD [75] Quantum dots Graphene [76] Graphene [78] Double layer tunneling junction BP GHz [20] Waveguide integrated BP ms [53] Field effect tuned Hybrid materials SWIR Graphene/PbS QD s [101] Photogate effect Graphene/PbSe/TiO ns [70] Graphene/PbS [23] growth methods of MBE, MOCVD, etc. [47,106] The formation of quantum confinement is usually achieved by using materials with mismatched conduction band or valance band. Figure 4a shows the schematic diagram of the conduction band alignment in a quantized system under external bias. The potential barrier at the interface by the band mismatch imposes a confinement to the free carriers in the well or dot layer. Upon light irradiation, photo-excited electrons that could overcome the confining potential barrier are swept out from the well or dot layer to the external contact. The intraband absorption in quantum wells and dots enables infrared photodetection far beyond the inherent bandgap limit of materials. To improve the absorption efficiency, they are often repeatedly stacked into multilayers in practical photodetector devices. Figure 4b illustrates the typical structure of a photodetector based on epitaxial quantum wells and dots. It is noteworthy that an optical coupling layer (gratings, corrugated layers, etc.) is often included in the photodetector design for an efficient light coupling to the absorber layer. [107,108] This is particularly important to quantum wells based photodetectors since 2D quantum wells will not directly absorb photons incident from the normal direction according to the selection rule. [109] However, this limitation doesn t exist for 0D quantum dots due to their three dimensional quantum confinements. [110,111] This feature rendering the feasibility of quantum dots based infrared photodetector in device integration. [112] Yet, due to the shape of quantum dots from Stranki-Krastanow growth mode usually deviate from the ideal round shape, the absorption of epitaxial quantum dots to normal incident photons remains weak. [113] Light coupling structures are thus still universally integrated in quantum dot based photodetectors. [37,114] Though the first QDIP was invented in 1998, [ ] great achievements have since been made in their performance, [119] functionalities, [120] and high resolution integration in focal plane arrays (FPA). [121] In experiments, epitaxial methods of 2D layered growth and 3D island growth modes are generally adopted to fabricate quantum wells and dots. In the 3D island growth, quantum dots are formed by the releasing of residual stress in thin films exceeding a critical thickness (Stranki-Krastanow growth). By using in-situ monitoring methods, such as the RHEED pattern, the growth stages of quantum wells and dots could be precisely controlled, allowing the intentional design of its optoelectronic properties in infrared photodetectors. So far, most quantum wells and dots are constructed based on III V compounds (In, Ga, Al: P, As, Sb) using the mature technologies developed for semiconductor lasers. With the more and more sophisticated techniques in controlling the growth rate, growth mode and dopant incorporations, many other photodetector designs have been brought in for optimized photodetection performances, such as dot-in-well (DWELL), type II superlattices, quantum cascade detectors (QCD). In Figure 4b, a schematic of the common dot-in-well structure is illustrated, in which the quantum dots are further embedded in well layers. Compared to sole quantum wells or dots, this structure renders more (6 of 26)

7 Figure 4. a) An illustration of the energy diagram alignment in quantum well and dot photodetectors. b) The typical structure of quantum well and dot based infrared photodetectors, including the multiple repeated well and dot layers, contact layers and an optical coupling layer. The structures of sandwich quantum wells and dot-in-well are indicated on the right. c) The widely tunable absorption spectra of GaN/AlGaN quantum wells by adjusting the width of well layer. Reproduced with permission. [122] Copyright 2007, IEEE. d) Multiple band photoresponse spectra observed in InAs/InGaAs/GaAs dot-in-well infrared photodetectors. Reproduced with permission. [123] Copyright 2009, IOP Publishing. flexibility in tuning the optoelectronic properties of photodetectors by tailoring the quantum confinement either in the dot or well layer. Although type II superlattices exhibit similarly tunable optical properties, they are constructed based on the efficient tunneling of carriers within adjacent well layers. Compared to traditional quantum well detectors, the interaction of carriers within nearby multiple layers enables additional tailoring the electron energy levels and hence their optical absorption. [124,125] Figure 4c,d show the spectral characteristics of different quantized systems of GaN/AlGaN quantum wells and InAs/ InGaAs/GaAs dot-in-well structure. [122,123] Due to the discrete energy levels under quantum confinements, their absorption is narrow band, in sharp difference with that observed in interband absorptions. Figure 4c further indicates the tunable absorption spectra of GaN/AlGaN quantum wells from SWIR to LWIR band, which is realized by simply adjusting the width of GaN layer. [123] In the case of dot-in-well, the presence of several electron energy levels in the dots, well and barrier layers leads to the multiband response spectra (Figure 4d) corresponding to each of the intraband transitions. [122] These featured photoresponse characteristics in quantum wells and dots based photodetectors have greatly promoted the feasible construction of multi-color sensitive infrared photodetectors for imaging purposes. However, it is necessary to mention that to reach efficient infrared intraband absorption in quantum wells and dots, the constituting semiconductor materials are often heavily doped to populate enough carriers in the ground energy states. As a result, quantum wells and dots based photodetectors often suffers high dark current, which is not desired for the integration in focal plane arrays with readout circuit. A lot of methods have been proposed to address this issue by using the well-developed device modeling and fabrication methods. For instance, resonant tunneling barriers have been introduced in devices to block the dark current from thermal excitation while allowing only the pass of photocurrent. [126,127] A thorough review of the various configurations of barrier based infrared photodetectors are recently provided by Martyniuk et al. [128] In addition to the barrier photodetectors, quantum cascade photodetectors with asymmetric cascade-like energy band alignments are also proposed for passive infrared photodetection using the photovoltaic mode. [95, ] They are initially developed in analogy to the principle of quantum cascade lasers. By avoiding the dark current, the operation temperature of these devices for MWIR detection could reach over 200 K. [130] Besides the quantum wells and dots, other multilayer structures have been also designed for infrared photodetection deep into the FIR and THz band. A series of study on heterostructured GaAs/AlGaAs had been conducted earlier by Liu et al. for THz detection, utilization the intraband transition of free carriers at the band offset of the junction. [135,136] Recently, Lao et al. reported a hot-carrier assisted infrared photodetection based on GaAs/Al x Ga 1-x As multilayers (Figure 5a). [137] As shown in the figure, the multilayered structure includes a graded barrier, an absorber layer and another constant barrier layer to form a band alignment shown in Figure 5b. Since GaAs has a bandgap of 1.42 ev, it is not expected to observe infrared responses in the LWIR and FIR band. However, such long wavelength detection is surprisingly achieved in this device by populating hot carriers into the absorber layer. Figure 5c shows the photoresponse spectra measured in the device when a short wavelength excitation (of a cutoff wavelength 4.5 µm) is applied simultaneously in the measurement. Under the short wave length stimuli, the photodetector starts to exhibit apparent photo response up to the wavelength of 50 µm. The stimuli in (7 of 26)

8 Figure 5. The structure a) and valence band alignment b) for a hot-carrier assisted infrared photodetector based on GaAs/Al x Ga 1-x As. c) Full photoresponse spectrum of the hot-carrier assisted infrared photodetector in the far infrared. d) The schematic illustration of hot-carrier injection effect to the optical transition in absorber layer. e) The spectra mapping of the responsivity of photodetector to the excitation power at 4.5 µm. Reproduced with permission. [137] Copyright 2014, Nature Publishing Group. this device are believed to inject hot carriers into the absorber layer by overcoming the graded barrier. With the gradual occupation of ground energy levels in the absorber layer by accumulated hot carriers, the absorption of lower energy infrared photons in the absorber layer becomes enhanced. This proposed photodetection principle based on the hot carriers in this photodetector is indicated in Figure 5d. Though the photon absorption in this structure is still originated from intraband transitions, it is however apparently different from quantum wells and dots due to the absence of quantum confinement. The essential role of the external light excitation in the observed LWIR and FIR photodetection is further identified from the observed excitation power threshold for the appearance of photo response in Figure 5e Solution Casted 0D Colloidal Quantum Dots Alongside the rapid growing quantum dot based infrared photo detectors, solution casted 0D colloidal quantum dots (CQDs) emerge as a novel candidate for high performance infrared detection. A variety of colloidal quantum dots that are sensitive in the broad infrared band have been fabricated using solution methods at considerably low cost, including PbS, [69,138] PbSe, [71, ] Ag 2 S, [141] HgTe, [32,42,142] HgSe, [36] etc. Due to the quantum confinement, they exhibit size dependent absorption cutoff wavelengths. In photodetectors, CQDs often display exceptional high photoconductive gains originated from the elongated carrier lifetime by the phonon bottleneck effect and surface traps. [ ] Accordingly, numerous research efforts have been devoted in understanding and engineering the carrier dynamics of CQDs in photodetector devices. [56,147] In contrast to the quantum dots from epitaxial growth methods, solution casted CQDs are fabricated as isolated particles without substrate support. They are dispersed in solvent after fabrication and assembled in devices in the way of close packing using methods of spin coating, drop casting, etc. The quantum effects in the colloidal quantum dots (CQDs) endow them size dependent bandgaps and thus similar tunable response spectra in photodetectors as epitaxial quantum wells and dots. However, due to the quantized energy levels in CQDs and the vast presence of surface and interfaces, the charge transport in CQDs films is often dominated by the hopping mechanisms, which is greatly different from conventional bulk materials. The rapid maturing synthesis of CQDs in defining their shape, size, composition and surface chemistry, [148] have greatly improved the ability within research community to design CQDs in terms of their absorption bands and electronic performances. [138,149] A prominent merit of CQDs in photodetectors is their high gain mechanisms compared to other nanostructures. [1] This is related to the capture of photoexcited electrons and holes in the trap states, which in turn elongated the carrier lifetime in CQDs to be much longer than the transit time. [54,56,147] PbS and PbSe colloidal quantum dots (CQDs) had been widely explored in infrared photodetectors and photovoltaics. Another attractive feature of quantum dots need to mention is related to the multi-exciton generation in quantum dots during light absorption. [150,151] This effect hold the promise to break the Shockley- Queisser limit in solar cells, [152] while in photodetectors, it offers a potential high photo-to-electron efficiency of more than unity. [146] With the direct band gap of 0.37 ev and 0.27 ev in the bulk form, they are sensitive in the NIR and SWIR bands for optical communication and bio-imaging. [9] In 2006, (8 of 26)

9 Konstantatos et al. reported the exceptional high detectivity of Jones at 1.3 µm using a thin film photodetector assembled by solution casted PbS CQDs. [56] The CQD thin films ( 0.8 µm thick) is simply coated on interdigitated electrodes. The high detectivity here is expected to be closely related to the long lived trap states up to ms in the native surface oxides on CQDs. It should be noted that the CQDs are generally stabilized after synthesis by insulating surface ligands. In the case of oleic acid, due to its long molecular chain of 2.5 nm, a postsynthetic ligand exchange using shorter ligands such as n-butylamine is necessary to ensure good electrical conductance in CQDs films. Though ultrahigh photoconductive gain could be achieved, the long lived trap states would however limit the photoresponse speed attainable in CQDs based photodetectors. To improve the response speed of CQDs based photodetectors, electrical field driven drift current is found desired to decoupling the carrier extraction from various photocarrier dynamics, including trapping, recombination and diffusion. In 2009, Clifford et al. reported PbS CQDs based Schottky diode device that could detect infrared light (1550 nm) at over 1.5 MHz. [54] In this device (Figure 6a), a 350 nm thin film of PbS CQDs is sandwiched between ITO and Al electrodes. A depletion region is formed in the device near the PbS/Al contact due to the presence of Schottky barrier (Figure 6b). Figure 6b further illustrates the various processes in extracting the photo current, including the drift component in the depletion region (DR) and the diffusion component in the quasi-neutral region (QNR) by hopping transport. In view of the different speeds of these two processes, the electrical field driven drift current is much desired over the slow diffusion current for high speed photodetectors. From such perspective, high speed photodetection in this device is realized by engineering the surface chemistry of CQDs in the way that increases the Schottky barrier height at the contact. The increased Schottky barrier is capable to extend the depletion region in the photodiode and hence the drift component in the overall photocurrent. In a fully depleted photodetector, a significant improvement of the response speed is achieved, reaching a 3dB frequency of over 1.5 MHz at the detection wavelength of 1550 nm while maintaining a high detectivity of over Jones. Benefited from the optimized carrier extraction dynamics, the external quantum efficiencies (EQE) in this device exceed 30%, as indicated in Figure 6c. Using external biases, ultrafast photocurrent dynamics within in ns was recently demonstrated. As mentioned before, the transient photocurrent response is often dominated by the slow carrier trapping and de-trapping processes. By applying an external bias, Gao et al. recently demonstrated a high speed CQDs based photodetector using sandwiched ITO/PbSe CQDs/Au structure (Figure 6d). [21] Using the external electric field, the photogenerated carriers in device are forced to separate and collect before they are trapped. Figure 6e,f illustrates the observed ultrafast photocurrent dynamics under pulsed excitation. The transit time of photocarriers in this device is found to fit well with the electric Figure 6. a) The structure of a Schottky diode photodetector based on PbS CQDs film. b) The diagram of energy band alignment in the diode, the important photocurrent extraction components by drift and diffusion processes is indicated together. c) The quantum efficiency and detectivity measured in the PbS CQDs based photodetector. Reproduced with permission. [54] Copyright 2009, Nature Publishing Group. d) The structure of a vertical stacked photoconductive detector based on PbSe CQDs. e) The ultrafast photocurrent dynamics and f) the extracted decay time in the photodetector under varied voltage biases. Reproduced with permission. [21] Copyright 2016, American Chemical Society (9 of 26)

10 field driven carrier transport (Figure 6f), with τ tran = L 2 /µv, where L is the transit distance, µ is the carrier mobility, V is the applied voltage. In this device, ultrafast photoresponse time 74 ps, EQE > 50% and responsivity of 0.36 A W 1 were obtained under a bias voltage of 1.3 V. It is noteworthy that such devices suffer high dark currents due to the pursuing of ultrafast speed. An overall improvement of CQD based IR photodetectors would rely on further optimization of the device structures, i.e., by introducing energy block layers. [153] Colloidal quantum dots of other narrow bandgap materials, such as HgTe, [32,42,142] HgSe, [36,97,154,155] have been recently synthesized for long wavelength infrared photodetection in the MWIR, LWIR and FIR bands. Figure 7a b show the TEM image and absorption spectra of HgTe CQDs. [32] The absorption by 7.1 nm and 10.5 nm HgTe quantum dots reaches 3 and 5 µm in MWIR. Remarkably, the photodetectors based on HgTe CQDs displays extraordinary photoresponses at room temperature (Figure 7c), which seem even comparable to the cooled MCT detectors. In the longer wavelength range of LWIR and FIR, Lhuillier et al. recently successfully fabricated HgSe CQDs with greatly expanded absorption band to even THz. This is realized by fully exploiting the intraband transitions in quantum dots. [36] Figure 7d,e show the absorption spectra of HgSe CQDs of different sizes. In Figure 7d, the characteristic bands of intraband absorption in CQDs appear around the wavenumber cm 1. By changing the size of CQDs, the infrared absorption by HgSe is seen to be continuously tuned from 3 to 20 µm. Moreover, the assembled photodetectors showed very promising photodetection performances at the wavelength of 6 µm, with a responsivity of 0.8 A W 1 and detectivity of over 10 8 Jones (Figure 7f). In spite of the marvelous achievements obtained in CQDs based infrared photodetectors, there are still huge room for further improvements from both the perspective of material quality control and device design. As mentioned in the above, due to the extraordinary high surface to volume ratio of CQDs, their surface chemistry plays an important role in determining their electronic properties and stability in devices. [140, ] Historically, organic ligands of octylamine, ethanedithiol, benzenedithiol have been widely used to modify the CQDs after synthesis. [9,140,159] However, recent findings indicate that inorganic capping (halide anions, lead halides perovskites) may provide competitive and even better passivation effects to the vast trap states in CQDs. [148, ] They showed elongated chemical stability up to several months. With the more and more sophisticated synthesis and surface modification techniques of CQDs, a continuous growing of their application in infrared photodetectors is expected D Nanowires and Nanotubes 1D nanostructures such as nanowires and nanotubes exhibit an inherent anisotropic structure. They have attracted lots of research interests in optoelectronic devices, including lasers, photodetectors, etc. [12,13,164] Due to the high surface to volume ratio, 1D nanostructures also accommodate similar photoconductive gain mechanisms in photodetectors as CQDs. Furthermore, the inherent anisotropic nature of 1D nanostructures goes a step further and renders a highway for carrier transport in devices and the feasibility in constructing polarization sensitive photodetectors. In this section, we survey the Figure 7. The TEM image a) and absorption spectra b) of HgTe CQDs, and their photocurrent response spectra c) in photodetectors. Reproduced with permission. [32] Copyright 2011, Nature Publishing Group. d),e) The infrared absorption of HgSe CQDs and f) the responsivity and detectivity of HgSe CQDs based photodetectors to 6 µm infrared light. Reproduced with permission. [36] Copyright 2016, American Chemical Society (10 of 26)

11 recent achievements in addressing the various challenges in constructing 1D nanostructure based photodetectors in the infrared spectrum. Narrow bandgap III V materials are of particular interests for infrared photodetection due to their ultrahigh mobility and direct bandgaps that fit for high speed detections. [165,166] Using MBE and CVD methods, InAs and InSb nanowires have been routinelly fabricated in laboratories. However, a common challenge for III V nanowires is that the universal native oxides on their surface tend to pin Fermi level above the conduction band, [ ] causing surface accumulation of free carriers. In photodetectors, this leads directly to significant surface leakage currents and low detectivity. Recent researches in nanowire infrared photodetectors have adapted to utilize Schottky contacts, axial heterojunctions and field effects to optimize their photodetection performance in infrared. Figure 8 shows the photodetection of InAs nanowire based photodetector working at the wavelength of 1.5 µm. [99] Single InAs nanowire was assembled in the device through lithography methods. From the inset of Figure 8a, it could be noticed that along the same InAs nanowire, different contacts of Cr and Au are prepared. Here, the adoption of Au electrode leads to the formation of Schottky contact due to the work function difference. The dark and photocurrent in Figure 8a and b correspond respectively to the measurements on devices with Ohmic- Ohmic and Schottky-Ohmic contact combinations. The energy band alignments in these two cases are find in Figure 8c. Due to the presence of built-in electrical field near the Schottky contact, a depletion of free carriers in InAs nanowire near the Au electrode is expected. Such depletion effect from contact works to apparently decrease the dark current in Schottky-Ohmic contacted photodetector. Moreover, the built-in electrical field near Schottky contact is found to also facilitate the charge separation efficiency in the photodetector. As a result, the responsivity ( A/W@1.5 µm) of InAs nanowire based photodetector is tripled by simply adopting the Schottky contacts in constructing the photodetector device. Instead using contact barriers, directly integrating rectifying junctions in nanowires offer an ideal solution to improve the on-off ratio in III V nanowire based photodetectors. From such perspective, researchers have been attempting to fabricate dopant-gradient nanowires. [24,25] Figure 9a displays the structure of a photodetector based on InAs 1-x Sb x nanowire array (Figure 9b), with the nanowires built directly with axial p-i-n junctions. The precise control of dopant incorporation is realized by vapor phase nanowire growth. In order to obtain the desired dopant gradient, the dopants atoms in nanowire are switched from Be to Te along the growth direction. As indicated in Figure 9c, the prepared p-i-n heterojunction in nanowire renders cascade-like energy band alignment. Hence, the photodetector based on the p-i-n junction in nanowires could be operated in photovoltaic mode with almost zero dark current (Figure 9d). Furthermore, since only a fraction of the substrate is covered with nanowires, the dark current in the photodetector based on nanowire array is further reduced from conventional vertical stacking devices. Because of these effects, the fabricated InAsSb nanowire photodetector were found to display apparent infrared response at room temperature. As shown in Figure 9e, the longest detection wavelength by the reported InAsSb nanowire based photodetector reaches 2.5 µm in SWIR band. Though the surface coverage of nanowires is low on substrate, it has been discussed that the light absorption efficiency in such vertical nanowire arrays may still be comparable to thin film devices due to the light trap effects. [171] The photogate effects that often appear in nanostructured photodetectors has been recently exploited to activate the infrared photodetection in InAs nanowires. [100] As explained earlier, in the photogate effect, the electrical conductance in nanostructure tends to be modulated by surface trapped charges through the similar field effects in phototransistors. Though photogate effect usually leads to additional photoconductive gains in photodetectors, it in some cases also causes negative photoresponses. In InAs nanwires, such negative photoresponse has been genearlly observed, in which the free carriers in nanowire tend to be depleted upon illumination by the photogate effects. [172,173] Taking advantage of such characteristic, Fang et al. recently reported an activation of MWIR response in single InAs nanowire based photodetector. Figure 10a shows the SEM image of the photodetector. Negative photoresponse in this nanowire is observed under 450 nm light irradiation, a wavelength that is much shorter than the absorption edge of InAs. The underlying mechanism of negative photoresponse is shown in Figure 10b. Under light illumination, the charges trapped in surface states depletes free carriers in nanowire and significantly modifies the potential barrier at the contact. As seen in Figure 10c, due to the presence of large dark current, Figure 8. Single InAs nanowrie based infrared photodetector. The dark and photocurrent in the detectors based on a) Ohmic-Ohmic and b) Schottky- Ohmic contact combinations. The energy band alignments in these two cases are displayed in c). Reproduced with permission. [99] Copyright 2014, American Chemical Society (11 of 26)

12 Figure 9. The infrared photodetector based on vertical InAsSb nanowires with axial p-i-n junction: a) the schematic configuration of the photodetector device. b) The SEM image of as grown InAsSb nanowire array on Si substrate. c) An illustration of the energy band alignment along the nanowire d) The dark and photocurrent in vertical InAsSb nanowire photodetector to 1.55 µm infrared light. e) The photoresponse spectra of InAsSb photodetector at 300K. Reproduced with permission. [24] Copyright 2016, American Chemical Society. the initial Ohmic contacted InAs nanowire using Cr electrodes shows no photoresponse to 2 µm infrared light. However, once a 450 nm light illumination is shed on the sample, the dark current in the photodetector is greatly suppressed due to its negative photoresponse to 450 nm excitation. An apparent infrared response at the wavelength of 2 µm could be finally observed in the photodetector after such an activation process (Figure 10d,e). It should be noted that to maintain the Figure 10. The negative photoresponse activated infrared photodetection in InAs nanowire photodetector. a) SEM image of the single nanowire photo detector. b) An illustration of the underlying mechanism of photogate induced negative response in InAs nanowire detector. The dark and photo current in InAs nanowire to 2000 nm infrared excitation c) before and d) after sheding the activation light. e) Photoresponse measurement indicated fast response speed of less than 100 µs in the device after activation. Reproduced with permission. [100] Copyright 2016, American Chemical Society (12 of 26)

13 photogate effect throughout the period of photodetection, the reported infrared photodetection is obtained at 77 K, given that the photogate effect would disappear more quickly at any elevated tempreatures. The decreasing of dark current in nanowire photodetectors could be also realized by exploiting external field effects. The foremost intention here is still to deplete the intrinsic carriers in nanowire channels. Recently, novel dielectric materials, such as ionic liquids, gels and ferroelectric polymers have been explored as novel dielectric materials in nanowire transistors and photodetectors. [98, ] Due to their relatively slow dynamic responses, the applied field effects in these materials could maintain for a moment in devices, rendering a continous modulation to the electrical conductance in nanowire channel. Such retention behavior in these dielectrics is inherently related to their slow ion diffusion and polarization rearrangement compared to the instaneous polarization response in conventional dielectric oxides. Though the retention tends to limit their switching speed in transistors, it has been recently exploited to construct fully depleted photodetectors with greatly suppressed dark current. Using ferroelectric P(VDF-TrFE) as the dielectric layer, Zheng et al. have realized a fully depleted nanowire photodetector, reaching an ultrahigh detectivity of Jones. [98] Once polarized, the remanant polarization field in the polymer is found maintain stable during the period of >10000 photodetection pulses. Besides narrow bandgap III V nanowires, 1D carbon nanotubes are also gaining many interests in infrared photo detection due to their broadband absorption. Utilizing the inherent anisotropic structure of carbon nanotubes, aligned nanotubes arrays have been use as polarization filters in front of photodetectors. [177,178] Though early attempts have been also made in directly constructing polarization sensitive photo detectors based on 1D nanowires, so far they are only demonstrated in the visible and NIR spectrum. [34,179] It would be interesting to see the appearance of polarization sensitive photodetectors in the infrared spectrum using the inherent anisotropic nature of 1D nanowires and nanotubes Graphene, Black Phosphorous and Other 2D Materials The recently emerging 2D materials represented by graphene, black phosphorus (BP) and transition metal dichalcogenides (TMDs) are attracting intensive attentions in photodetectors. [17, ] These materials exhibit atomic thick layered structures and only weak van der Waals interactions between neighbor layers. Due to the strong out of plane quantum confinement, the optoelectronic properties of 2D materials are prone to be modified by the number of layer stacked and their stack sequence. [10] For example, the bandgap of BP is tuned from 0.35 ev in bulk to over 1 ev for a single layer. [186] The atomic thin thickness of 2D materials also resolves them excellent tolerance to external strains, making the 2D materials especially attractive as the building blocks in flexible electronics. [ ] Moreover, the electronic properties of 2D materials are found feasibly tuned by field effects and strains, [ ] which could be exploited for their engineering in photodetector devices. For infrared photodetection, graphene and black phosphorus are mostly explored because their broadband absorption to low energy infrared photons. They are often fabricated into ultrathin flakes by exfoliation methods from bulk crystals, using Scotch tape peeling or liquid phase intercalation methods. [195,196] For the construction of electronic devices, the 2D flakes are further transferred to arbitrary substrates using various wet and dry methods. [ ] Alternatively, many 2D materials (graphene, TMDs, etc.) are now able to be obtained directly on substrates from chemical vapor deposition (CVD). [ ] Due to the absence of bandgap, graphene exhibits an extremely broadband absorption from UV to THz. [77,78, ] It is found that despite of the atomic layer thickness, graphene absorbs 2.3% of incident photons without significant wavelength dependence. [207] In photodetectors, its absorption efficiency could be improved by stacking multilayers or by engineering graphene into other nanostructures, such as nanodisks. [206,208] With the enormous carrier mobility (up to cm 2 V 1 s 1 ) and ultrafast carrier dynamics ( ps), [ ] graphene based photodetector are possible to work at the extremely high frequency of 500 GHz. [19] To reach an effective carrier extraction in such short time scales, various junctions like metal-graphene junction, graphene homogeneous p-n junctions, tunneling barrier junctions and Van der Waals junctions are developed to facilitates the separation of photo-excited carriers in graphene. [17,180] The ultrathin nature of graphene makes it very sensitive to ambient environment, surface chemistry modifications and external electrostatic fields. This characteristic is feasibly harnessed to modify the electronic properties of graphene, making it into n-type or p-type. Due to the change of surface absorbents, graphene from mechanical exfoliation was found to experience a gradual transformation from p-type in ambient to n-type when placed in vacuum conditions. [214] In sight of this phenomenon, surface chemical modification methods has been widely adopted for the controllable n- or p-type doping in graphene layers. [ ] A graphene p-n junction photodiode has been demonstrated based on polymer coating at the selected area of graphene. [27] A schematic configuration of the photodetector by this strategy is displayed in Figure 11a. Using external field effects, graphene p-n junctions are also directly made by applying dual gate modulations. [26,218,219] In planar graphene photodetectors, it has been identified that most of the photocurrent are collected near the junction area using scanning photocurrent mapping technique. [26] This is related to the ultrafast carrier dynamics in graphene and reduced charge separation efficiency outside the space charge region. To improve the collection efficiency of photo-excited carriers, a high performance detector thus demands a maximum junction area within the device. This could be improved by using closely packed interdigitated electrodes, [220] or adopting vertical junctions in photodetector. [28] Figure 11b shows the schematic structure of a vertical junction based on graphene. In addition to the above mentioned maximum area of photocurrent generation in devices, the vertical junctions also render more efficient separation of photocarriers by ultimately reducing the transit distance of carriers. However, due to the Klein tunneling transport, [221,222] rectifying behaviors are fundamentally not observed in vertically stacked p-n junctions of graphene. Instead, the graphene layers in (13 of 26)

14 Figure 11. a) The schematic of a photodetector based on planar graphene p-n junction. Reproduced with permission. [27] Copyright 2014, American Chemical Society. b) The schematic of a photodetector based on vertical tunneling junction of graphene, c) illustration of the separation mechanism of photocarriers in the tunneling junction. d) The examined photocurrent under infrared light (3.2 µm). Reproduced with permission. [78] Copyright 2014, Nature Publishing Group. e) The photoresponse in all-graphene based vertical tunneling diode. Reproduced with permission. [78] Copyright 2014, Nature Publishing Group. devices need to be separated by thin tunnel barriers made by BN, Ta 2 O 5 etc. [78,223] Using such tunneling junctions, Liu et al. recently demonstrated a broadband photodetector (Figure 11b) that are sensitive in MWIR based on graphene. [78] Figure 11c illustrates the photodetection mechanism in such tunneling junction. The top and bottom graphene layer exhibit shifted Fermi levels (estimated to be 0.12 ev) due to the different extents of interaction with substrates. As a result, the photogenerated electrons and holes under illumination tend to be separated by tunneling to the opposite layer. By floating the top layer during photodetection, photogenerated charges are accumulated in the top layer and induce the photogate effects to the bottom channel. Interestingly, it has been found that the photodetection spectra of such device could be engineered by adjusting the tunneling barrier height. Using narrow bandgap Si as the barrier layer ( 0.5 ev above the fermi level of intrinsic graphene), the authors were able to demonstrate the room temperature detection of MWIR ( 3.2 µm) with the high responsivity of 1.1 A W 1 (Figure 11d). Instead of using arbitrary stacked barrier layers, Kim et al. found that through chemical doping methods, an insulate layer could be induced in graphene in the vertical direction. This phenomenon is further utilized for the fabrication of all-graphene based vertical tunneling junctions. [28] When made in photodetectors, clear rectifying behavior and broad photoresponse characteristics from UV to NIR were observed (Figure 11e), whereas it is not clear yet the upper-limit of the detection wavelength of such photodetector. Similar to graphene, black phosphorus (BP) is also an elemental layered material, but instead has an intrinsic direct bandgap of 0.35 ev. The small bandgap is especially suitable for broadband photodetection deep into the MWIR. Compared to graphene, the moderate bandgap in BP allows high on-off ratio switching in field-effect transistors and low dark current in photodetectors. [ ] The interaction between layers renders layer number dependent bandgaps in few layer BP, which could be tuned from 0.35 ev to 1 ev. [228] Figure 12a illustrates the layered orthorhombic structure of BP, which is anisotropic along the armchair and zigzag direction. [35] Recently, using exfoliated BP, Guo et al. successfully constructed a mid-infrared sensitive photodetector that works at room temperature. [53] Figure 12b shows the schematic of BP based photodetector assembled using interdigitated electrodes. The BP thin film used in this device has the thicknesses of nm and a bandgap similar to bulk BP, which enables MWIR detection. Figure 12c displays the measured photoconductive gain and responsivity in the photodetector at various incident light powers at the infrared wavelength of 3.39 µm. By using external field effect modulation, the device was able to reach a high responsivity of 82 A W 1 at room temperature with the high photoconductive gain of over It is noteworthy that the high photoconductive gain in the BP photodetector could maintain at the frequency of up to 1 khz, limited by the response speed of photogate effects. Moreover, due to the anisotropic nature of BP, BP based photodetector devices were demonstrated with apparent polarization sensitivity at the MWIR band (Figure 12d). More prominent (by a factor of 2) photocurrent was found along the armchair direction (indicated as xx in the figure). Other recent literatures also demonstrate that such structural anisotropy induced polarization sensitivity could be routinely achieved in BP based photodetectors. [35,53,229,230] From the perspective of high speed detection, Youngblood et al. recently were able to demonstrate ultrafast BP based photodetectors working at the frequency of exceeding 3 GHz (Figure 12e) and at the telecommunication band of 1.55 µm. [20] This is realized by avoiding the slow photoconductive gain mechanisms in BP by engineering the electrostatic doping (14 of 26)

15 Figure 12. a) The orthorhombic crystal structure of black phosphorous. Reproduced with permission. [35] Copyright 2015, Nature Publishing Group. b) An infrared photodetector based on 2D BP using interdigitated electrodes, c) its responsivity and gain to 3.39 µm infrared light under varied incident light power. d) The anisotropic photocurrent response observed in BP based photodetectors. Reproduced with permission. [53] Copyright 2015, American Chemical Society. e) Ultrafast photoresponse realized in electrostatic doping modulated BP photodetector. Reproduced with permission. [20] Copyright 2015, Nature Publishing Group. The rapid emerging of other 2D materials, i.e., TMDs, have renders the advanced construction of multiple types of van der Waals heterojunctions. [ ] The different types of heterojunctions have shown the potential to manipulate the carrier separation dynamics in photodetectors. However, due to their relative large bandgap of 1 2 ev, their applications in photodetectors are so far mainly explored in the UV-Vis and NIR band. [10,180,182] Nevertheless, they can still play vital roles in engineering the electronic properties of existing infrared sensitive graphene and black phosphorus. [235,236] With the rapidly developed growth, transfer and stacking technologies of 2D materials, the enormous emerging of infrared photodetectors based on various 2D material combinations are expected Hybrid Heterostructures To reach a comprehensive performance optimization of a photodetector, including the cutoff wavelength, bandwidth, responsivity and detectivity, hybrid heterostructures are often employed in the construction of photodetectors. By forming type-ii band alignments, heterostructures are capable to achieve a natural separation of photogenerated carriers, avoiding the collaborated construction of p-n junctions and Schottky junctions. Moreover, the unique merits of each constituting materials could be integrated in the same photodetector for an optimized performance. For example, ultrahigh photoconductive gain of 10 8 have been demonstrated in a photodetector that combines the high mobility of graphene and large extinction coefficient of colloidal quantum dots. [23] In this section, we summarize the recent achievements in hybrid heterostructured photodetectors specialized for the infrared detection. The prominent advantage of heterostructured photodetectors in that photogenerated electrons and holes could be separated directly at the interface if proper band alignment is formed. This could be realized by choosing the materials with type II band offsets. [23,101,235, ] Taking the recent 2D materials as the example, Figure 13 shows the various types of heterostructured photodetectors in literature. Similar constructions can be made using 1D nanostructures or thin films. [241,242] For 2D graphene, BP and others, due to the ultrathin thicknesses, their overall infrared absorption efficiency in devices is still low ( 2.3% absorbed in monolayer graphene). In sight of this, various heterostructure combinations had been introduced in experiments to sensitize 2D materials. The sensitizers could be simply decorated or directly mixed with 2D materials, as displayed in Figure 13. Considering the feasible assemble methods, high extinction coefficient materials in the forms of quantum dots, [23,101] nanosheets/platelets and organic dye mole cules have been frequently used. [70,239,243,244] Figure 13a shows the schematic of a PbS CQDs/graphene based photodetector that reaches remarkable responsivity of (15 of 26)

16 Figure 13. The variety of heterostructure combinations in photodetectors based on 2D material: a) PbS CQDs/Graphene, (reproduced with permission. [23] Copyright 2012, Nature Publishing Group) b) Bi 2 Te 3 nanoplatelets/graphene, (reproduced with permission. [239] Copyright 2015, American Chemical Society), c) organic dye/mos 2, (reproduced with permission. [243] Copyright 2014, American Chemical Society), d) three-body-system of PbS/TiO 2 /Graphene, (reproduced with permission. [70] Copyright 2012, Wiley-VCH). The inset in (a) illustrates the charge separation at the interface of graphene and PbS CQDs, which in turn induces photogate effect to the graphene layer A/W in the NIR band. [23] In this combination, the light absorption occurs in the colloidal quantum dots while the charge transport is conducted in the high mobility graphene. [23,101] Due to the band misalignments (inset of Figure 13a), the photo generated electrons and holes in PbS CQDs are separated near the interface: holes are injected into graphene, while electrons are left behind in the CQDs films. This in turn imposes a strong photogate effect that modulates the electrical conductance in graphene layer, reaching a gain of 10 8 electrons per absorbed photon. In this hybrid structure, the spectral response could be steadily tuned to the short-wavelength infrared by choosing the appropriate sizes of CQDs. In the last few years, tremendous emerging of similar heterostructured infrared photodetectors has been seen, including Ge/graphene, [55] Bi 2 Te 3 /graphene, [239] PbSe/MoS 2, [240] BP/MoS 2, [235] etc. In addition to the heterostructures with two constituting components, Manga et al. investigated the three-bodysystem of Graphene/PbSe/TiO 2 that have multiple interfaces (Figure 13d). [70] The prepared composite exhibit high extinction coefficient of >10 5 cm 1 in a broadband from visible to SWIR (<2 µm). The assembled device not only showed high detectivity of over Jones in the NIR and >10 12 Jones at 1400 nm, but also kept surprisingly fast response time of less than µs. This is ascribed to the efficient carrier separation and extraction in the composite by the ambipolar electron and hole transport. This result clearly indicates the huge potential in managing the carrier dynamics in heterostructured materials for a comprehensive performance enhancement in photodetectors. Semiconducting polymers exhibits various special advantages in devices by their low cost and the feasibility in large scale fabrication and integration. However, their absorption cutoff wavelengths are usually limited in the NIR. To extend their response spectra in photodetectors, different kinds of hybrid heterostructures have been explored, including allorganic and inorganic-inorganic combinations. [57,245] Figure 14 shows the infrared photodetectors based on PbS CQDs and semiconductor polymers of PCBM and P3HT (electron and hole accepting materials). In the device, PbS CQDs are simply dispersed in the organic absorber, which is sandwiched by top and bottom contacts (Figure 14a). As seen in Figure 14b, the photoresponse in this device reaches 1.8 µm when using larger PbS CQDs. The simple fabrication method of polymer blend and CQDs enables the simple design of integrated photodetector arrays and imaging sensors that work in the SWIR band. The tunable response cutoff wavelength by CQDs could further render a feasible development of multi-color photodetectors in this spectra range. The unique properties associated with heterostructure interfaces have also been explored to construct photodetectors that are rewritable. Using the well-known two dimensional electron gas (2DEG) in the interface of LaAlO 3 /SrTiO 3, Irvin et al. were able to demonstrate a rewritable photodetector that are sensitive in the NIR (Figure 15). [246] As illustrated in Figure 15a, this is realized by applying a local electrical field to the heterostructured interface from a conductive-afm tip. In this device, an insulator to metal transition could be forced at the interface by using a specified electrical field that drives the formation of quasi two-dimensional electron gas (q-2deg). The conductance could in following be erased by using a reversed electrical field. A nanowire is thus intuitively written in the device using a line scan, while it is further made into a nanogap through a second cross line scan with reversed electric field. In Figure 15b,c, the photocurrent in this device is apparently only generated near the perimeter of nanogap. This finding certainly reveals the (16 of 26)

17 Figure 14. a) Schematic of the structure of an infrared imaging device based on PbS CQDs:P3HT:PCBM hybrid materials and b) the measured photoresponse spectra when CQDs of different sizes are used. Reproduced with permission. [245] Copyright 2009, Nature Publishing Group. broad potential of various heterostructures in constructing optoelectronic devices through any imaginable methods, though the demonstrated responsivity is still low (Figure 15d). As seen in the above, in the past few years, lots of progresses have been achieved in exploring heterostructured nanostructured materials in photodetectors. Yet, this is not coming to the end as long as novel materials and phenomena are continuously being discovered and applied to photodetectors. A synergistically engineered heterostructure after thorough understanding of the fundamental carrier dynamics would undoubtedly further leverages the photodetector performances in the future. This would however rely intimately on the precise tailoring of the interfaces in heterostructured hybrid materials. 4. Surface Plasmon Resonance Enhanced Infrared Photodetectors With the overall detection performance of nanostructured photo detectors more and more approaching the intrinsic limit set by the material themselves, many attentions have been paid to enhance the light-matter coupling in photodetectors. The surface plasmon-polaritons (SPPs) resonance that resides at the conductor-dielectric interface has recently received many interests from this perspective. The unique properties of surface plasmons in resonance absorption, subwavelength confinements and near field enhancements have been extensively explored in photodetectors in the past few years for the optimization of their performances Infrared Absorption by Surface Plasmon Excitation Surface plasmon or plasmon-polaritons (SPs or SPPs) depicts the electromagnetic wave that are trapped at the metal-dielectric interface by the collective oscillation of free electrons. [247] In the direction normal to the interface, the electromagnetic fields of SPs decay exponentially into the metal and dielectric medium, rendering a subwavelength confinement to light waves. In the meantime, SPs are capable to propagate along the interface Figure 15. A rewritable photodetector based on LaAlO 3 /SrTiO 3 heterostructure, the electrical conduction at the interface is tuned to be conductive by the specific electrical field applied in conductive AFM. b) The photocurrent mapping in the written nanowire junctions, the photocurrent collected when the illumination spot is place at and outside of the junction area are shown in c). d) The photoresponse spectra of the rewritable photodetector. Reproduced with permission. [246] Copyright 2010, Nature Publishing Group (17 of 26)

18 and be used to guide light wave paths. [248] Since SPs could be excited by light waves with matched frequency and momentum, the unique features of SPs lead to a wealth of opportunities in electronic and photonic devices. [249] In isolated nanoparticles and some artificially designed metamaterials, SPPs become localized and their excitation leads to the damping and absorption of light in the medium. In chemical sensors, photovoltaics and catalysis, the localized surface plasmon resonance on metal nanoparticles have been extensively explored to exploit their characteristics in resonance absorption and near field enhancements. [250,251] In photodetectors, the surface plasmon resonance induced hot electrons could be directly harness for photodetection. [93,94,252] In this section, recent literature within this scope will be discussed. Historically, SPs in Ag and Au metal nanoparticles and thin films had been extensively explored because of their resonance frequencies in the visible spectrum. [253,254] This is intimately related to their high carrier concentration of over cm 3. In experiments, the resonance frequency of metal nanostructures could be shifted by engineering their geometry and surrounding dielectric environments. [255,256] Using various nano-fabrication methods, such as EB lithography, focused ion beam milling, nanoimprinting and chemical synthesis routes, a precise design of SP nanostructures is now possible. [251] This is further facilitated by the guidance from accurate simulation tools (finite difference time domain method, etc.). By tuning the sizes and geometries, the resonant frequencies of Au nanostructures have been routinely moved to the NIR band. Figure 16 shows the surface plasmon resonance assisted NIR photodetector based on bilayer MoS 2 and Au nanostructured electrodes. [93] The photodetector utilizes the high photoconductive gain from MoS 2 and the plasmonic NIR resonance absorption in Au electrode. As seen in Figure 16a,b, asymmetric geometries of Au electrodes are designed in the photodetector to support different resonances of surface plasmon. Under a resonant light excitation, the hot electrons from resonant wire (RW) could be injected to MoS 2 for photodetection by overcoming the Schottky barrier at the contact interface. Figure 16c indicates the NIR photoresponse in the photodetector under varied bias conditions. Due to the 1.65 ev bandgap of MoS 2, it exhibits no absorption in this band. It is found that the photoresponse within nm is maximum when the bias polarity matches with the injection and extraction of hot carriers (Figure 16d f). This phenomenon validates the role of surface plasmon induced hot electrons in the observed photodetection. In the broadband infrared beyond NIR, to exploit the surface plasmon resonances, novel materials have to be utilized instead of metals. Semimetals and heavily doped semiconductors (ITO, etc.) with carrier concentrations of cm 3 have been found to exhibit surface plasmon resonances in the SWIR spectrum. [94,257] Through a modified colloidal chemistry method, Zhu et al. were able to engineer semimetallic TiS 2 into nanoplatelets, which could accommodate surface plasmon resonances in the telecommunication band ( 1500 nm). [94] Figure 17a illustrates the 2D layered structure of TiS 2 and the prepared platelets-like structure. Taking advantage of the localized surface plasmon resonances in the nanoplatelets, the absorption spectrum of TiS 2 was remarkably extended compared to the bulk materials, as seen in Figure 17b. The surface plasmon resonance induced infrared absorption in this structure was further explored to enhance the infrared detection based on PbS CQDs. Recently, the plasmon modes in 2D graphene are attracting intense interests due to their resonance frequencies in the infrared spectrum. [ ] Researchers have been attempting to utilize the plasmonic resonance of free carriers in graphene to enhanced the light interaction in thin graphene layers. Recently, Freitag et al. reported a photodetector (Figure 18a) based on graphene nanoribbons (GNRs, Figure 18b). [258] The infrared sensitive plasmonic resonance in GNRs are exploited for long-wavelength infrared detection at 10.6 µm. Such graphene nanoribbons, like other nanodisks and rings, [208,259] support the standing surface plasmon modes that could remarkably enhance the infrared light interaction within graphene Figure 16. a) The schematic of a NIR photodetector based on surface plasmon resonance induced hot electrons. b) Simulated electric field distribution at the resonance wavelength. c) The photo-responsivity of the photodetector in the NIR band under various bias voltages. d) f) illustrates the hot electron induced photocurrent under positive, negative and zero bias. Reproduced with permission. [93] Copyright 2015, American Chemical Society (18 of 26)

19 Figure 17. 2D TiS 2 semimetal nanoplatelets from colloidal chemical synthesis method. a) The schematic of 2D layered crystal structure of TiS 2 and the morphologies of TiS 2 nanoplatelets. b) The surface plasmon resonance enhanced absorption spectra of TiS 2 nanoplatelets. Reproduced with permission. [94] Copyright 2016, Wiley-VCH. (Figure 18c). Depends on the geometry, chemical or electrostatic doping conditions, the plasmon resonance frequency in graphene nanostructures can be further modulated largely in the infrared spectrum. [ ] Intriguingly, the polarization dependence of surface resonance in such anisotropic GNRs enables a polarization detection of incident infrared lights. Figure 18d displays the polarization dependence of photocurrent in this device. Enhanced photocurrent by the excitation of the plasmon resonance is found along the direction of s-polarization, which is perpendicular to the nanoribbon axis. Figure 18e shows the scanning photocurrent mapping under this polarization. From the comparison of photocurrents along s- and p-polarization, it is estimated that the photoresponse in this device is enhanced by 15 fold from the exploitation of surface plasmon resonances in graphene. Compared to the localized surface plasmon resonances in the visible and NIR spectrum, the infrared sensitive plasmon resonances in materials are far from understood due to their possible interaction with polar phonons in materials. Exploring and engineering their properties in devices are likely to reinforce the current infrared photodetectors from a viewpoint Near Field Enhancements and Waveguides Instead of direct absorption, near-field enhancement effects in surface plasmons are more often employed in devices to improve the light absorption in materials. [38,42,114,263] This can greatly improve the photodetector performance to low energy photons that tends to be poorly absorbed. In the meantime, many efforts have been devoted to utilize the propagating surface plasmons to guide far-field incident light waves to nanoscale photodetectors beyond the diffraction limit of optical lenses, [247,264,265] which could be potentially realized by the subwavelength confinement of electromagnetic waves at the metal-dielectric interface. To reach an efficient coupling of light to SPs on metal surface, corrugated textures, gratings, periodic metal disks or holey Figure 18. The infrared photodetector based on graphene nanoribbons (GNRs). a) The schematic of the photodetector device. b) The SEM image of GNRs defined by EB lithography and oxygen plasma etching. c) The extinction spectrum of the GNRs on SiO 2 substrate. d) Polarization dependent photocurrent and e) the scanning photocurrent mapping in the photodetector. Reproduced with permission. [258] Copyright 2013, Nature Publishing Group (19 of 26)

20 metal films are usually necessary. [247] This is determined by the momentum mismatch between light wave in air and the SPs at the metal-dielectric interface. Due to the rapid decay of evanescent waves into metal and dielectric medium, in photodetectors, the active absorber need to be placed within the proximity of interface for light absorption. [263] Plasmonic dots or disk arrays have been utilized to optimize the light absorption efficiency in colloidal quantum dot photodetectors and 2D graphene based photodetectors. [42,220,266] The resonance feature of plasmonic nanostructures also enables the construction of narrow band infrared detectors. A recent report by Tang et al. showed such a detector based on HgSe colloidal quantum dots with integrated Au disk arrays, reaching 2 5 times enhancement to the responsivity in MWIR and LWIR. [97] In the different case of epitaxial quantum well infrared photodetectors, 2D hole arrays (2DHAs) made in metal thin films are recently widely explored for an efficient light coupling in device. Due to its period structure in two dimensions, a full photonic bandgap for SPs is created, while near the band edge the high SP states are able to support intense field enhancements. [247] Using various lithography methods, the geometry and periods of 2DHAs can be routinely designed to fit for the requirements of different target wavelengths. [38,263] Figure 19a shows the schematic of a quantum dot-in-well (DWELL) photodetector with 2DHAs light coupler integrated on its top. [37] The active detection element in this photodetector is consisted of 20 stacks of InAs/In 0.15 Ga 0.85 As layers. The simulation of SP field (Figure 19a) indicated intense field enhancement near the perimeter of holes. The photoresponse improvement observed in the photodetector by the enhancement of light coupling displayed very sharp spectral resolution of 10% (Figure 19b). Besides, it has been demonstrated that such spectral selectivity can be further transformed into polarization sensitivity by designing the periodic structures of 2DHAs, e.g., by selectively stretching the lattice constant in a specified direction of 2DHAs. [114] In the last decade, the feasibly engineered SPs characteristics in metal thin films has intrigued tremendous efforts in constructing various SP nanostructures supported multi-color and polarization sensitive photodetectors. [31,38,41,263] For 3D nanostructured photodetectors, specific SPs structures are needed to fit the light coupling to nanostructures beyond planar films. Recently, Lee et al. reported a partial metal cover that accommodate near-field enhancements to the photodetector based on InAsSb nanopillar (NP) arrays (Figure 19c e). [267] As mentioned earlier, NW or NP based photodetectors exhibit much lower dark currents compared to planar devices. However, their light absorption relies on the light trapping in the array structure by multi-reflections. As result, when the light wavelength becomes larger than the sizes (diameter, length) of NW or NP, the light coupling efficiency drops considerably due to the diffraction limit. In the report by Lee et al., with the specially designed plasmonic nanostructure, they were able Figure 19. a) Schematic of a photodetector with integrated 2DHA light coupler and the simulated plasmonic field distribution in the detector. b) Plasmonic resonance enhanced photoresponse in the detector. Reproduced with permission. [37] Copyright 2011, Nature Publishing Group. c) The schematic of nanopillar (NP) photodetector with partial covered metal layer to support near field enhancement. d) Simulated electric field distribution in 3D NP photodetector at the resonance wavelength. e) The photoresponse spectra of the photodetector, the peak at 2390 nm is ascribed to SPs induced near-field enhancement. Reproduced with permission. [267] Copyright 2016, American Chemical Society (20 of 26)