Compound Quantum Dot Perovskite Optical Absorbers on Graphene Enhancing Short-Wave Infrared Photodetection

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1 Compound Quantum Dot Perovskite ptical Absorbers on Graphene Enhancing Short-Wave Infrared Photodetection Alexander A. Bessonov,,* Mark Allen, Yinglin Liu, Surama Malik, Joseph Bottomley, Ashley Rushton, Ivonne Medina-Salazar, Martti Voutilainen, Sami Kallioinen, Alan Colli, Chris Bower, Piers Andrew, Tapani Ryhänen Emberion Limited, Sheraton House, Castle Park, Cambridge CB3 AX, United Kingdom Emberion y, Metsänneidonkuja 8, Espoo 13, Finland Hybrid graphene photodetector operation. As optical absorbers, colloidal quantum dots (QDs) can be tailored to absorb light of certain targeted wavelengths determined by the semiconductor bandgap and nanocrystal size, but have relatively poor carrier mobilities (1 3 to 1 cm /V s) limiting their application in photodetection. Graphene, in contrast, exhibits exceptionally high mobility ( > 1 3 cm /V s is feasible in practise) and its conductance is very sensitive to electrostatic perturbation. Combining quantum dots with graphene field-effect transistors (GFETs) creates photoconducting devices exhibiting gate-controlled current modulation (i.e. they are phototransistors). 1 QDs close to the graphene surface of a GFET modulate the charge carrier density in the graphene channel when illuminated. This occurs through the formation of electron-hole pairs upon photon absorption and subsequent charge separation at the QD-graphene interface. The quantum efficiency (η) of the process of a photon being absorbed and the subsequent photo-induced carrier being trapped has been experimentally approximated to be ca. η 5% for small-size (< 4 nm in diameter) PbS QDs passivated with 1,-ethanedithiol (EDT) ligands. 1 The built-in field between the QDs and graphene (due to the work function mismatch) allows the excited holes to transfer to the graphene channel while electrons remain trapped in the QDs. 1 Depending on the biasing conditions or absorber property affecting the band alignment, this can be reversed so that holes remain trapped and electrons pass to the channel. The trapping time (τ trap ), also referred to as trap-limited carrier lifetime, depends on the intrinsic properties of the QDs and can span from sub-milliseconds to seconds. If the trapped charges are electrons, the trap charge density induces positive charges in the GFET channel through capacitive coupling; this change in charge carrier density is observable as a change in the GFET channel resistance. Since the carrier transit time (the time taken for a hole to traverse the channel at the carrier drift velocity) is of the order of τ transit 1 1 ns for typical channel lengths, 1 photo-gated GFETs with modulated resistance will exhibit a huge change in the overall number of charge carriers passing through the device during τ trap (namely, the gain G = τ trap τ transit ). S1

2 a 1 m b c Figure S1. Graphene field-effect transistor arrays on plastic. (a) Microscope image of the GFET array comprising 16 pixels. (b) Photograph of the device showing mechanical flexibility. (c) Self-assembled monolayer (SAM)-treated GFETs on polymer substrates operate stably under electrical bias, with minimum current deviations (recorded for a single GFET). S

3 a b CH 3 CH 3 CH 3 CH 3 P P P P c Figure S. Surface morphology analysis. (a, b) Atomic force microscopy (AFM) images of selectively binding n-octadecylphosphonic acid (DPA) SAM on Al3 and (c) 6.3 nm in diameter PbS QD film on graphene obtained by solid-state ligand exchange with pre-mixed PbI and MAI. RMS roughnesses are.5 nm and 3.7 nm, respectively. Highly uniform DPA SAM coatings with excellent ambient stability reduce the oxide surface energy (static water contact angles >1 ) and mask surface charges. S3

4 Current ( A) Resistance (k ) Responsivity (AW -1 ) a b c GFET 1 GFET Gate voltage (V) Current ( A) GFET 1 GFET Current ( A) GFET 1 GFET Figure S3. Effect of hysteresis on GFET switching. (a) Exemplary transfer curves of GFET devices without (GFET 1) and with hysteresis (GFET ). The samples are fabricated on a silicon substrate at different conditions (dehydration and encapsulation steps). Gate voltage scan rate is 1 V/s, V ds =.1 V. (b, c) Gate modulation of the hysteretic device (GFET ) either side of the Dirac point, results in drifting and slow relaxation of the current flowing through the channel (I ds ). The hysteresis-free GFET 1 device, in contrast, demonstrates an immediate and stable response to gate biasing. The data show that the hysteresis in GFET performance can be detrimental to high-speed operation of hybrid graphene photodetectors. a b Pixel number Figure S4. Multi-pixel performance. (a) Transfer characteristics of 16 devices in the QD-GFET array fabricated on dual-stacked graphene, showing multi-pixel consistency. The leakage current through the gate is measured to be less than 1 pa. The corresponding current density through the gate dielectric is less than 1 fa μm. The drain-source voltage is V ds =.1 V, the data are measured in the dark. (b) Responsivity values for a typical array device functionalized with PbS QDs treated by MAPbI3. Recorded for diode laser pulses of 5 nm wavelength ( ), the light intensity of 1.1 W m, the pulse frequency of.1 Hz, the optimum gate bias point V g V Dirac 1.5 V, and V ds =.1 V. S4

5 PbS EDT PbS EDT E VAC E C E F Graphene Graphene E V 3. GFET with PbS EDT GFET unsensitized 8 V g = V -8 Resistance (k ) n-type shift I ph /I (%) R/R (%) Figure S5. EDT-capped large-size (6.3 nm in diameter) PbS QDs on GFETs: General structure, band diagram, graphene doping analysis, and temporal photoresponse. The energy band alignment with p-doped graphene is similar to the PbS QDs treated by MAPbI3, however, a weaker and slower response is observed, attributed to incomplete QD surface passivation leading to deep traps and numerous recombination centers. A moderate shift towards negative gate voltages occurs in GFETs functionalized with PbSEDT (dehydrated and encapsulated). The light intensity is 1.1 W m, = 5 nm, the optimum gate bias point V g V Dirac 1.5 V, and V ds =.1 V. S5

6 PbS QD exciton peak 13 nm N H stretch C H stretch N H bend 1565 C H bend 1465 Absorbance (a.u.) As synthesized Aged 1 days in solution Aged 1 days in film Wavelength ( m) Wavenumber (cm -1 ) Figure S6. Infrared absorption of unencapsulated MAPbI3 PbS films. The data shows the disappearance of the excitonic peak (13 nm) for the film aged under ambient conditions, whereas it is preserved when stored in n-butylamine solution (left panel). Methylammonium signatures (N H, C H) in the FTIR are not affected by aging (right panel). This indicates a displacement of the energy bands in the QDs relative to the perovskite matrix upon exposure to ambient light and moisture, and is consistent with expectations for type- II and quasi-type-ii systems with progressively increasing electron-hole separation. 3k Counts k 1k MAPbI 3 MAPbI 3 -PbS PbS MAPbI Raman shift (cm -1 ) Figure S7. Raman spectroscopy of optical absorbers. Raman measurements at 53 nm laser wavelength suggest full degradation of the perovskite matrix yielding the most stable orthorhombic form of Pb, known as massicot. The spectra obtained (bands at 86, 143, 88 cm 1 ) are in agreement with those of massicot reported in the literature.,3 Although PbS is known to be a weak Raman scatterer, no apparent oxidation of PbSMAPbI3 is observed under laser illumination, in contrast to MAPbI3 and MAPbI3 PbS. S6

7 a.5 Resistance (k ) Gate voltage (V) b c 4 5 light on light off -5 light on light off I ph /I (%) R/R (%) I ph /I (%) R/R (%) W m W m Figure S8. Performance of MAPbI3 PbS sensitized devices. (a) Transfer characteristics of GFETs with MAPbI3 PbS. Pointers (1) and () indicate the gate voltages either side of the Dirac point (.5 V and +.5 V), at which photocurrents of opposite polarity were recorded. (b, c) A substantial change in GFET channel resistance (5 33% of R R) on illumination can be achieved using MAPbI3 PbS absorbers. Photoresponse is measured under a single pulse of 5 nm laser diode (1.1 W m ),.1 Hz pulsing frequency, V ds =.1 V. S7

8 Responsivity The charge density in the light absorbing film can be modelled using a Taylor series expansion of the nonlinear rate equation as n t ηφ opt n n ξn 3 νn 4 (1) τ trap where η is the quantum efficiency, Φ opt the initial photon flux, τ trap the trap-limited carrier lifetime, and γ, ξ, ν are coefficients related to electron-hole pair recombination rate. Under the steady state condition the transfer function from the photon flux to trap charge can be written as n Φ opt η 4νn 3 + 3ξn + n + 1 τ trap () where the carrier density n is a solution of the steady state equation νn 4 + ξn 3 + n + n τ trap = ηφ opt (3) The photodetectors based on GFET with a light absorber are significantly non-linear compared to photodiodes. In order to calculate the noise characteristics of the device correctly, it is important to define the responsivity in the point of operation differentially 6,7 as R = ( I ds ) = ( Φ opt ) ( n ) ( n t P opt P P opt Φ opt n ) ( I ds eμv ds ) n t opt (hc λ)l ( n ) (4) Φ opt where P opt is the optical power, I ds the drain-source current, n the number of trapped carriers in the absorber layer, n t the total number of charge carriers in the graphene channel, μ the carrier mobility in the graphene channel, e the elementary charge, hc λ the photon energy, and L is the channel length. The responsivity can be solved numerically from the previous equation, or it can be approximated with the following intuitive equation based on the same derivation: R eμv ds (hc λ)l ητ trap 1 + (4τ trap γηφ opt ) α (5) Thus, the responsivity of hybrid photodetectors can be described as R R max 1 + (4τ trap γηφ opt ) α (6) eη where R max = (hc λ) τ trap, τ τ transit = L (7) transit μv ds S8

9 Here an exponential correction constant α is used to correct the effect of higher order nonlinearities. The intuitive equation is based on an assumption that the first and second order terms in the rate equation (1) are more significant than the higher order terms. Taking into account only the first and second order terms, it is possible to find the solution (5) with α = 1. Figure S9 shows the measured responsivity and model fittings for devices with PbSEDT and MAPbI3 absorbers (see the main text for PbSMAPbI3). The fitting parameters are shown in the following table. PbS MAPbI3 MAPbI 3 PbS EDT γ m /s m /s m /s a Responsivity (AW -1 ) 1 5 MAPbI 3 PbS EDT Linear regime Responsivity (AW -1 ) b Current ( A) 47.3 light on light off Current ( A) Photogating reversal light on light off 1 1 MAPbI 3 PbS Irradiance (Wm - ) Irradiance (Wm - ) Figure S9. Responsivity of sensitized graphene phototransistors. (a) Responsivity versus irradiance for GFETs with PbSEDT and MAPbI3. V ds =.1 V. The dots represent experimental data, whilst the solid lines represent the fourth-order nonlinear model given by equations (), (3) and (4), and the dash lines are the solutions of the equation (5). The non-linearity of responsivity (and detectivity) as a function of irradiance originates from electron-hole recombination becoming dominant with increasing excitation intensities. At low intensities, the charge carrier density in the QDs is small, and the steady-state photoconductivity is governed solely by trapping thus demonstrating a linear regime of operation. (b) Responsivity versus irradiance for GFETs with hybrid MAPbI3 PbS, where a reproducible reversal of the photocurrent polarity occurs at the irradiance between 11 and 83 mw m. V ds =.1 V, V g =.5 V, λ = 5 nm. Light-governed transformations suggest that it is favorable to trap holes at high light intensities (n-type photogating) whereas electrons are predominantly trapped when the intensity decreases (p-type photogating). Practically, it is possible to avoid the effect by having a proper feedback to stabilize the operating point of the device in place, by tuning the graphene Fermi level with respect to the absorber Fermi level. S9

10 Noise performance The performance of the graphene transistors is fundamentally limited by the 1 f noise of the drain-source current. 4 1 f noise is known to be related to complex interactions between random processes, such as impurity and grain boundary scattering, charge fluctuations, thermal fluctuations, etc. Graphene transistors have several potential sources of 1 f noise, such as charge puddling, scattering by impurities, grain boundary and contact resistance fluctuations. In addition, the absorber material in the vicinity of graphene can contribute to charge fluctuations in the transistor channel. The experimental results of several groups, including our own measurements (see Fig. S1), show similar a scaling law for the 1 f noise. The power spectral density of the current noise is S1 f = βids Af where I ds is the drain-source current, A the gate area, f the frequency, and β is an experimental parameter that is related to the quality of the graphene layer. The RMS noise can be derived as I n,rms = βi ds A ln (f high ) f low where the noise bandwidth Δf = f high f low. In our present case, the highest frequency f high is determined by the cutoff frequency of the preamplifier, and the lowest frequency f low is given by the light chopping frequency or the exposure time. The noise equivalent power is defined as NEP = I n,rms R. The specific detectivity at varied light intensities and exposure times is calculated using Example of detectivity calculation: D = WL f NEP 1. Photocurrent is recorded as a change of the current in the dark and under illumination, e.g. ΔI ph = I light I dark = 63.4 A 6.6 A = 1.14 A for V ds =.1 V, V g = 3 V, λ = 5 nm, the optical power density of 3.34 mw m, and the chopping frequency of.1 Hz.. Responsivity is taken as R = ΔI ph P opt = A W 1, with the optical power P opt = 5.41 pw (GFET area W L = 9 18 m). 3. NEP is calculated as I n,rms R = W Hz 1/, where the noise current I n,rms = 5 na, measured in the dark and integrated over a 1 khz.1 Hz bandwidth at the operational gate bias point. 4. Detectivity is then D = WL f NEP = Jones (cm Hz 1/ W 1 ) for the bandwidth f of 1 khz. S1

11 a b c Dual-layer GFET with PbS MAPbI3 Single-layer GFET with PbS MAPbI3 Single-layer GFET with MAPbI S i /I xwxl ( m Hz -1 ) Resistance (k ) S i /I xwxl ( m Hz -1 ) Resistance (k ) S i /I xwxl ( m Hz -1 ) Resistance (k ) d Noise spectral density S I (A Hz -1 ) Frequency (Hz) e Noise current I n (na) QD-GFET GFET Figure S1. Noise analysis. (a, b) Low-frequency 1 f noise spectral density of sensitized GFETs as a function of the gate bias, showing minimum noise at the Dirac point, consistent with previous reports. 4 Noise level is reduced in dual stacked graphene (a) compared to a single layer (b), in a similar way observed for bilayer graphene. 4 Presumably, improved contacts may play a role in the noise reduction. 5 (c) Perovskite absorbers result in nearly flat noise spectral density with respect to gate bias, owing to strong p-doping of graphene. The sampling frequency is 1 Hz, V ds =.1 V. (d) Noise spectral density S i as a function of frequency, recorded on the dual stacked graphene devices with the QDs at the gate bias ranging V g = 5 +5 V and V ds =.1 V, resulting in I ds range of 1 3 A, for which the noise was averaged. The 1 f noise at the operating point is approximately A Hz 1/ at 1 Hz; and 1 9 A Hz 1/ at 1 Hz. Note that the noise level largely scales with the drain-source current. A typical shot noise related noise floor is 1 11 A Hz 1/, a thermal noise floor is A Hz 1/. (e) RMS noise current integrated over the bandwidth 1 khz 1 Hz for GFETs with and without MAPbI3-treated PbS QDs (encapsulated devices, recorded for different samples). The data are measured in the dark. S11

12 There exists a tradeoff between the cut-off frequency and the detectivity of the hybrid graphene photodetector. We define the cut-off frequency as the point where the responsivity has decreased 3 db from its low frequency value, and we use the optical cut-off frequency as the measurement frequency. The noise bandwidth is determined by the optical and electrical cut-off frequencies, f optical,cutoff = 1 τ trap and f e,cutoff, respectively. Assuming that the 1 f noise of the graphene channel is dominating, we can write a tradeoff equation as ητ D trap 4n (hc λ) f e,cutoff 1/(τ trap ) α ln (f e,cutoff τ trap ) where η is the quantum efficiency, n the residual charge density of the graphene channel, α the amplitude parameter of 1 f noise, λ the wavelength, and τ trap is the charge trapping time constant. In Fig. 7 (see the main text) we have used the typical measured values to calculate the tradeoff curve: η =.5, λ = 5 nm, n = 1 1 cm, α = 1 7 m, and f e,cutoff = 3 khz. References 1. Konstantatos, G.; Badioli, M.; Gaudreau, L.; smond, J.; Bernechea, M.; García de Arquer, F. P.; Gatti, F.; Koppens, F. H. L. Hybrid Graphene Quantum Dot Phototransistors with Ultrahigh Gain. Nat. Nanotechnol. 1, 7, Burgio, L.; Clark, R. J. H.; Firth, S. Raman Spectroscopy as a Means for the Identification of Plattnerite (Pb), of Lead Pigments and of Their Degradation Products. Analyst 1, 16, Bell, I. M.; Clark, R. J. H.; Gibbs, P. J. Raman Spectroscopic Library of Natural and Synthetic Pigments. Spectrochim. Acta, Part A, 1997, 53, Balandin, A. A. Low-Frequency 1/f Noise in Graphene Devices. Nat. Nanotechnol. 13, 8, Karnatak, P.; Sai, T. P.; Goswami, S.; Ghatak, S.; Kaushal, S.; Ghosh, A. Current Crowding Mediated Large Contact Noise in Graphene Field-Effect Transistors. Nat. Commun. 16, 7, Gupta, M. S. Thermal Fluctuations in Driven Nonlinear Resistive Systems. Phys. Rev. A 1978, 18, Gupta, M. S. Thermal Noise in Nonlinear Resistive Devices and Its Circuit Representation. Proc. IEEE 198, 7, S1