Hybrid Graphene Quantum Dot Devices and Their Applications

Transcription

1 Hybrid Graphene Quantum Dot Devices and Their Applications Eric Law Department of Electrical and Computer Engineering University of California, Davis Davis, CA Term Paper for EEC 247: Advanced Semiconductors Devices, Spring

2 Contents 1 Introduction 3 2 Fundamentals and Device Physics High Mobility Carriers as Massless Dirac Fermions Quantum Confinement, Coulomb Blockade Effect, Landau Level, and the Quantum Hall Effect Single Electron Transistor Responsivity Different Variations of These Devices Graphene Single-Electron Transistor Graphene Film with QD All Carbon-Based Device Structure and Fabrication 8 5 As Phototransistors/Photodetectors and Various Sensors Phototransistor/Photodetector SET-Based Charge Sensor, Photoluminescence, and Electrochemical Sensors. 11 2

3 1 Introduction Graphene has attracted strong scientific and technological interest in recent years due to its unique electronic properties, making it a promising candidate for electronics applications, and in particular, nanoelectronics. Graphene is a two-dimensional sheet of carbon atoms, which opens up a possibility for ever-smaller devices without the drawback of short-channel effects in conventional MOSFETs. Graphene itself is a semi-metal with zero bandgap (unlike a semiconductor such as silicon), but has a very high electron mobility and lifts the limit of current silicon-based transistors due to short-channel effects with its potential to be extremely thin. On the other hand, quantum dots and nanoparticles have also attracted immense interest among researchers because these they often exhibit drastically different yet desirable properties than their macroscopic counterparts of the same material. For example, both gold and copper nanoparticles have been reported to have conductivities of 10 7 times smaller than gold and copper in bulk. Quantum dots could also substantially increase the density of electronic components. For example, single electron transistors (SETs) have been realized with CdSe quantum dots [1]. By combining the two materials and concepts together, it is possible to create graphene films with foreign quantum dots, or hybrid graphene-based quantum dot (GQD) devices that have similar or even better performances in terms of charge mobility, photon gain, density, lower cost, responsivity as IR detectors, and other possible parameters which will be discussed more in depth in later sections. The paper is structured as follows: The origins of those highly desirable and unusual characteristics will first be discussed, such as the device physics pertaining to graphene, followed by the many different types of these devices that research teams from around the world have developed. We will also explore the fabrication techniques used to manufacture these devices, and lastly their existing and potential applications. 3

4 2 Fundamentals and Device Physics 2.1 High Mobility Carriers as Massless Dirac Fermions As mentioned in the introduction, graphene exhibits unusually high carrier mobility which could be explained in the following manner. Electronic properties of materials are commonly described by quasiparticles that behave as non-relativistic electrons with a finite mass and obey Schrödinger s equation. However, graphene displays unusual characteristic where its charge carrier mimics relativistic particles with zero-mass known as massless Dirac fermions.[2] These massless Dirac fermions have effective speeds that are close to that of the speed of light, and thus resulting in fast electron mobility in graphene. However, The fabrication of graphene nanodevices have been deterred by the difficulty in confining these massless Dirac fermions due to Klein tunneling and the zero-band-gap electronic structure. 2.2 Quantum Confinement, Coulomb Blockade Effect, Landau Level, and the Quantum Hall Effect Moriyama s team from the International Center for Materials Nanoarchitectonics in Japan has reported an approach to confine carriers in a single-layer graphene, which could lead to quantum devices with field-induced quantum confinement.[3] They present an experimental demonstration of a field-induced Coulomb blockade effect and quantum confinement in the graphene device. In their experiment, graphene mesoscopic islands are perfectly isolated and metallic contacts are directly deposited onto them. This allows direct contact to the mesoscopic 2-D electron gas (2DEG) system. The confinement they presented is induced by both a uniform magnetic field perpendicular to the graphene sheet and an electrostatic surface-potential formed by the metal/graphene junction of their device. In Graphene, because each carbon atom contributes one π electron and each electron may occupy either a spin-up or spin-down state, the valance band (or π band) is completely filled 4

5 and the conduction band (or π band) completely empty. The Fermi level is there situated at the points, known as Dirac points, where the π band touches the π band.[4] Near the Dirac point, conductance is strongly suppressed by the magnetic field. In addition, several resonance peaks appear in the hole and electron carrier regions. In particular, clear resonance peaks are observed in the hole-carrier region (V g V Dirac ). These peaks correspond to the Coulombblockade effect. Diamond-shaped regions appear in the hole-carrier regions, which indicates that the current is suppressed due to Coulomb blockade effect. Under a perpendicular magnetic field, a Landau level (LL) generally forms in the 2DEG system. Landau levels are cyclotron orbits of discrete energy values that charged particles can occupy under the influence of a magnetic field. The LL energies E N of the single-layer graphene sheet are given by E N = sng(n)v F (2e h N B) where e is the fundamental unit of charge, where h is the reduced Planck s constant, v F is the Fermi velocity, and the integer N is the electron (N > 0) or a hole (N < 0) LL index. Note that the N 1 = LL (E 0 ) is independent of magnetic field in a single-layer graphene; i.e., E 0 is fixed at the Dirac-point energy with the magnetic field. However, the N = 0 LL spectrum does not appear in the device, which is attributed to the fluctuating spatial-disorder potential near the Dirac point and the pn p band profile. Another team led by Suyoug Jung from the Center for Nanoscale Science and Technology in Maryland also studied the localization of the Dirac fermions in realistic devices. They studied these interactions using scanning tunneling spectroscopy (STS) of exfoliated graphene on a SiO 2 substrate in an applied magnetic field. Jung s team took STS measurements of a gated single-layer exfoliated graphene device in magnetic fields ranging from zero to the quantum Hall regime.[5] In the quantum Hall regime, the Hall conductance begins to take on quantized values. With this ability to control the charge density of these Dirac fermions using magnetic fields, the team was able to observe that at zero magnetic field, weakly localized states are 5

6 created by the substrate induced disorder potential, and at higher magnetic fields, the 2DEG breaks into a network of interacting quantum dots formed at the potential hills and valleys of the disorder potential. In other words, the magnetic field strongly affects the electronic behavior of the graphene, and the properties of graphene are a direct function of the disorder potential. 2.3 Single Electron Transistor The single electron transistor is worth mentioning as a fundamental concept in these devices that exploit quantum mechanical phenomena. An SET is a structure in which electrons are spatially confined. Tunneling barriers connect the SET weakly to source and drain leads. The addition of each extra electron to the SET requires a classical charging energy e 2 /C (C is the quantum dot s capacitance). This leads to a ladder of discrete addition levels µ N indicating the energy required to add the N th electron. These levels can be shifted up or down in energy by applying a voltage to the plunger gate of the SET. At low temperatures (k b T e 2 /C) and a given finite source drain biasvoltage, current can only flow, if one of these levels is shifted through the bias window by sweeping the plunger gate voltage. If no level is in the bias window, the current is blocked as a result of the Coulomb interaction between electrons (Coulomb blockade effect).[6] 2.4 Responsivity One motivation for the research and fabricated of graphene with foreign quantum dots is to improve the responsivity of graphene as detectors. Graphene has an extremely high theoretical carrier mobility of up to cm 2 V 1 s 1 ; however, graphene has a very low responsivity as IR detectors. ( 6.1 maw 1 ) By modifying the graphene film with QDs, which absorb IR light more efficiently, the responsivity could be improved substantially. Such device will be discussed in the next section along with different variations of graphene/qd devices. 6

7 3 Different Variations of These Devices 3.1 Graphene Single-Electron Transistor Though not exactly a hybrid device, the so-called single electron transistor (SET) has been realized with graphene by Ihn s group from the Solid State Physics Laboratory in Zurich, Switzerland.[6] SETs consist of a small sub-micron sized island coupled weakly to source and drain contacts and takes advantage of the high mobility of graphene. Figure 1 shows a schematic drawing of a typical nanostructure made from a monolayer graphene flake. 3.2 Graphene Film with QD As alluded earlier, graphene film with QD devices have been studied to enhance the responsivity of graphene as IR detectors. In particular, phototransistors with ultrahigh responsivity up to 10 7 AW 1 based on mechanically exfoliated single or bilayer graphene flakes and PbS QDs modified with ethanedithiol were reported [7]. PbS is a semiconductor with a bandgap of 0.41 ev and an ionization potential of 4.95 ev when it is a bulk material. However, the ionization potential of PbS QDs is drastically higher due to quantum confinement effects. For PbS QDs, the bandgap is estimated to be 1.2 ev, and the conduction and valence bands of PbS QDs are estimated to be 4.15 ev and 5.35 ev, respectively. Figure 2 shows the schematic drawing for charge generation at a PbS QD/graphene heterojunction under light illumination. Another way to enhance the responsivity of graphene is proposed by Chang-Hua Liu s group at the Department of Electrical Engineering and Computer Science, University of Michigan, by constructing an ultra-broadband photodetector composed of two graphene layers sandwiching a thin tunnel barrier. The trapped charges on the top graphene layer can result in a strong photogating effect on the bottom graphene channel layer, yielding an unprecedented photoresponsivity over an ultra-broad spectral range. [8]. This method eliminates the light absorption being reliant on the quantum dots instead of the graphene, which would restrict the spectral range of 7

8 photodetection otherwise. Figure 3 shows the schematic drawing of this device structure. 3.3 All Carbon-Based Device Yet another variation of a hybrid GQD device and attempt was by Shih-Hao Cheng s group at the Department of Physics, National Taiwan University, where the team developed an all-carbon device consisting of graphite QDs, and two dimensional graphene crystals.[9] A remarkable responsivity of 4x10 7 AW 1 was obtained and was attributed to the spatial separation of photogenerated electrons and holes due to the charge transfer caused by the appropriate band alignment across the interface between graphite QDs and graphene. Figure 4 shows the schematic of their all carbon-based photodetector. 4 Structure and Fabrication For the graphene film with PbS QD mentioned above, chemical vapor deposition (CVD) was used to fabricate the single-layer graphene. The graphene was prepared on copper foils by the CVD and transferred to the substrates. The single-layer graphene was characterized by Raman spectroscopy to show the monolayer property. Raman spectroscopy is a technique used to observe vibrational, rotational, and other low-frequency modes in a system and relies on Raman scattering. A single-layer of graphene on a Si/SiO 2 substrate before and after being coated with PbS QDs capped with pyridine was observed under atomic force microscopy (AFM). Figure 5 shows AFM images of the single-layer CVD-grown graphene and the PbS QDs modified graphene films. The roughness of the graphene film is about 0.25 nm. The film becomes very rough after being coated with QDs and the roughness is about 2.7 nm. QDs on the surface of a graphene film were observed under transmission electron microscopy (TEM). Figure 6 shows the TEM image of the PbS QDs modified graphene film. It was found that the average size of the PbS QDs is 4 nm and their distribution on graphene is relatively uniform.[7] 8

9 Figure 7 shows the structure of a graphene photoconductor fabricated on a nsi/sio 2 substrate. The device also can be thought of as a field-effect phototransistor if highly doped silicon is used as the gate electrode. Gold source and drain electrodes were deposited on top of the patterned graphene film by thermal evaporation through a shadow mask. The field-effect mobilities are 1000 cm 2 V 1 s 1 for both electrons and holes and are much higher than those of conventional semiconductors used in the industry. Then PbS QDs capped with pyridine were coated on the graphene film by dropping the QD toluene solution on the surface. The device was dried for several hours in a glovebox filled with high purity N 2 before further measurements. Figure 8 shows the transfer curve of the graphene transistor modified with PbS QDs and measured in the absence of light. Noteworthy, the transfer curve becomes asymmetric and the Dirac point shifts to a positive gate voltage ( 50 V) after the modification, indicating p-type doping in the graphene film. In addition, the electron mobility decreases to <440 cm 2 V 1 s 1 while the hole mobility remains unchanged.[7] Similarly, the method used by Liu s group was also through CVD-grown graphene on copper foil and transferred to Si/SiO 2 substrate. Raman spectroscopy was also used to confirm the single-layer nature of the graphene. To fabricate their graphene/ta 2 O 5 /graphene heterostructures, they first transferred a graphene film onto a degenerately p-doped silicon wafer with 285 nm thermal oxide. Photolithography, graphene plasma etching and metal lift-off processes were used to fabricate the bottom graphene transistor. The sample was then covered by a 5-nm-thick Ta 2 O 5 film as the tunnel barrier, blanket-deposited by radiofrequency sputtering. Finally, the top graphene layer was transferred on top of the Ta 2 O 5 thin film. Similar to the bottom layer, photolithography, graphene etching and metal lift-off processes were used to produce the top graphene transistor. To fabricate their graphene/silicon/graphene heterostructures, the same procedure was used except Ta 2 O 5 was replaced with 6 nm intrinsic silicon film deposited by sputtering. 9

10 The graphene sheet in Cheng s all carbon-based photodectors were also made on copper foils by using CVD because this method allows for a large scale production of graphenebased devices with a high yield. As for the fabrication of the graphite QDs, 2 grams of 11- aminoundecanoic acid was first dissolved in 25 ml deionzied water and neutralized by 1 M sodium hydroxide aqueous solution. 2 grams of citric acid was also added. The precipitation that follows was collected by filtration and dried at 85 C for one day. Finally the QDs are extracted y hot water after it has been oxidized in air at 300 for 2 hours. To create the interface between the graphite QDs and the graphene films, the QDs are spin-casted onto the graphene film by instilling the graphite QD drops into the surface. The spin-speed was set to 1000 rpm throughout the film preparation, moved into a glove-box filled with high purity N 2, and baked for 100 for two minutes to improve the contact between the two. 5 As Phototransistors/Photodetectors and Various Sensors 5.1 Phototransistor/Photodetector One potential application of the hybrid GQD devices mentioned above is its use as phototransistors in detectors. A phototransistor is a type of photodetector consisting of a transistor channel with an optically controlled gate. Under optical illumination, optically-excited carriers generated in the top layer can tunnel into the bottom layer, leading to a charge build-up on the gate and a strong photogating effect on the channel conductance. These devices demonstrated room temperature photodetection from the visible to the midinfrared range, with mid-infrared responsivity higher than 1 AW 1, as required by most applications. These results address key challenges for broadband infrared detectors, and are promising for the development of graphene-based hot-carrier optoelectronic applications. A team led by Gerasimos Konstatatos at the Institute of Photonic Sciences was able to demonstrate a gain of 10 8 electrons per photon and a responsivity of 10 8 AW 1 in a hybrid photodetector that consists of monolayer or bilayer graphene 10

11 covered with a thin film of colloidal QDs.[10] 5.2 SET-Based Charge Sensor, Photoluminescence, and Electrochemical Sensors On the other hand, GQDs are also used in SET-based charge sensors. SETs utilize controlled electron tunneling to amplify a current. When both the gate and bias voltage of the GQD-based SETs are zero, electrons do not have enough energy to enter and current will not flow. As the bias voltage between the source and drain is increased, an electron and pass through and current can flow, which means the detection of charge can be realized.[11] GQDs prepared through different methods can emit photoluminescence (PL) with different color. Up to now, deep UV, green, yellow, and red PL of GQDs have been reported. The first PL sensor based on GQDs was developed by Jin et al in They found that the fluorescence of GQDs synthesized by periodic acid oxidizes could be selectively quenched by Fe 3+ ions sensitively and selectively through the charge transfer process. GQDs can also be used to build electronic sensors for the detection of humidity and pressure; GQDs that are selectively interfaced with polyelectrolyte microfibers form an electrically percolating-network that exhibit changes in their conductivities as a function of humidity and pressure, which results from changes in the tunneling widths due to the induced water transport in the hygroscopic microfibers. In addition to their applications as charge sensors, GQDs are widely used as a kind of novel electrode material in fuel cells, supercapacity batteries, photovoltaic cells, and electrochemical sensors. Such an application as an electrochemical/biosensor has been shown by Li et al, using GQD modified pyrolytic graphite electrodes coupled with specific sequence of single-stranded DNA molecules as probes. GQDs/Au electrode was also demostrated to be able to detect H 2 O 2 in living cells. Last but not least, electrochemiluminescence (ECL) emission has been observed 11

12 from GQDs as well, which show promise in ECL biosensors. 12

13 Figures Figure 1: Schematic of a typical nanostructure made from a monolayer graphene flake [6] Figure 2: Schematic for charge generation at a PbS/graphene heterojunction under light illumination [7] 13

14 Figure 3: Schematic for device structure of Liu s ultra-boardband photodetector composed of two graphene layers sandwiching a thin tunnel barrier [8] Figure 4: Schematic of Cheng s all carbon-based photodetector [9] 14

15 Figure 5: AFM images of the single-layer CVD-grown graphene, and the PbS QDs modified graphene films [7] Figure 6: TEM images of the PbS modified graphene film [7] 15

16 Figure 7: Schematic of the structure of a graphene photoconductor fabricated on a nsi/sio 2 substrate [7] Figure 8: Transfer curve of the graphene transistor modified with PbS QDs and measured in the abscence of light [7] 16

17 References and Notes 1. Goesmann, H. and Feldmann, C. (2010), Nanoparticulate Functional Materials. Angew. Chem. Int. Ed., 49: doi: /anie Novoselov, K.S.(2005), Two-Dimensional Gas of Massless Dirac Fermions in Graphene. Nature 438, doi: /nature Moriyama, S. (2014), Field-Induced Confined States in Graphene. Appl. Phys. Lett. 104, doi: / Goerbig, M.O. (2011), Electronic Properties of Graphene in a Strong Magnetic Field. Rev. Mod. Phys. 83, doi: /revmodphys Jung, S. (2011), Evolution of microscopic localization in graphene in a magnetic field from scattering resonances to quantum dots. Nature Physics 7, doi: / nphys Ihn, T. (2010) Graphene Single-Electron Transistors. MaterialsToday Volume 13, Issue 3, doi: /S (10)70033-X. 7. Sun, Z., Liu, Z., Li, J., Tai, G.A., Lau, S.P. and Yan, F. (2012), Infrared Photodetectors Based on CVD-Grown Graphene and PbS Quantum Dots with Ultrahigh Responsivity. Adv. Mater., 24: doi: /adma Liu, C. (2014), Graphene photodetectors with ultra-broadband and high responsivity at room temperature. Nature Nanotechnology 9, doi: /nnano Chang, S. (2013), All Carbon-Based Photodetectors: An eminent integration of graphite quantum dots and two dimensional graphene. Scientific Reports 3, Article number: doi: /srep

18 10. Konstantatos, G. (2012), Hybrid Graphene-Quantum Dot Phototransistors With Ultrahigh Gain. Nature Nanotechnology 7, doi: /nnano Sun, H., Wu, L., Wei, W., Qu, X. (2013), Recent Advances in Graphene Quantum Dots for Sensing. MaterialsToday Volume 16, Issue 11, doi: /j.mattod