Ultrafast Zero-Bias Photocurrent in GeS Nanosheets: Promise for

May 15, 2017 - Design of new photovoltaic systems based on two-dimensional group-IV monochalcogenides for high performance solar cells. Pei Zhao ...
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Ultrafast Zero-bias Photocurrent in GeS Nanosheets: Promise for Photovoltaics Kateryna Kushnir, Mengjing Wang, Patrick Devin Fitzgerald, Kristie J. Koski, and Lyubov V. Titova ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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Ultrafast Zero-bias Photocurrent in GeS Nanosheets: Promise for Photovoltaics Kateryna Kushnir1, Mengjing Wang2, Patrick D. Fitzgerald1, Kristie J. Koski3 and Lyubov V. Titova1* 1 Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, USA 2 Department of Chemistry, Brown University, Providence, Rhode Island 02906, USA 3 Department of Chemistry, University of California Davis, Davis, California 95616, USA *[email protected] Abstract: Ferroelectric semiconductors have been predicted to exhibit strong zero-bias shift current, spurring the search for ferroelectric semiconductors with bandgaps in the visible range as candidates for so-called shift current photovoltaics with efficiencies not constrained by the Schockley-Queisser limit. Recent theoretical works have predicted that 2D IV-VI monochalcogenides are multiferroic and capable of generating significant shift currents. Here we present experimental validation of this prediction, observing ultrafast shift currents by detecting terahertz electromagnetic pulses emitted by the photoexcited GeS nanosheets without external bias. We explore excitation fluence, orientation and excitation polarization dependence of the terahertz emission to confirm that shift currents are indeed responsible for the observed emission. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third generation photovoltaics based on shift current, or bulk photovoltaic effect. Graphical abstract:

The bulk photovoltaic effect (BPVE) can occur in any material that does not have inversion symmetry. The prevailing mechanism behind BPVE is a zero-bias photocurrent, or shift current, as excitation of an electron from the valence to the conduction band results in a spatial shift of the electron charge density on the order of a lattice constant and subsequent ballistic quantum coherent carrier transport.1-7 The possibility of a zero-bias photocurrent without the need to 1 ACS Paragon Plus Environment

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form a p-n junction makes materials that exhibit strong BPVE attractive candidates for new types of solar cells with efficiencies not constrained by the Shockley-Queisser limit.8-10 Hot shift current carriers can rapidly travel to electrodes where they can be collected, provided that required travel distance is comparable to their mean free path which is estimated to be on the order of 10-100 nm.6 Nanostructuring materials that exhibit shift current and embedding them into current-collecting matrix with suitable work function may result in optimal BPVE device efficiency. Shift current is particularly pronounced in ferroelectric materials. Lattice distortion that is responsible for a non-zero intrinsic, spontaneous polarization also results in a broken inversion symmetry.7 As a result, ferroelectrics can exhibit shift current response to unpolarized excitation light, unlike non-polar non-centrosymmetric materials that can only generate shift currents when excitation is linearly polarized.7, 9, 11-12 Most prototypical oxide ferroelectrics such as BiFeO3 have wide bandgaps which limit the portion of solar spectrum they can absorb. Novel perovskite materials with bandgaps in the visible and near-IR range have been recently suggested as possible candidates to circumvent this limitation.13-14 Finally, yet another class of narrow bandgap ferroelectric materials composed of earthabundant elements may offer another route for implementing third generation photovoltaics. Recent theoretical studies predict that monolayer van der Waals monochalcogenides such as GeSe, GeS and SnS are multiferroic with coupled ferroelectricity and ferroelasticity, and are thus promising candidates for BPVE applications.15-16 Figure 1a shows the top view (x-y plane) of an orthorhombic group IV monochalcogenide monolayer. Group IV monochalcogenide monolayers are noncentrosymmetric and are predicted to exhibit spontaneous polarization coupled with an elastic distortion as top and bottom atoms in armchair structure shift in x-y plane either along x direction (shown) or y direction.17-18 With high polarization and moderate predicted bandgap (~1.8 -2.3 eV for a monolayer),19-20 GeS is a particularly promising material in this regard, and it recently has been predicted to exhibit a large shift current as optical excitation results in an instantaneous spatial shift of electron density distribution along S-Ge bond (Figure 1a).18, 21 Using ab-initio tight binding calculation within a two-band model, Cook et al predicted that the shift current in monolayer GeS has a large peak at the band edge (~1.89 eV), and then falls as 1/ at higher frequencies.18

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Figure 1. (a) Top view of the structure of GeS monolayer exhibiting spontaneous distortion in x direction, and schematic 18 depiction of an in-plane shift current due to an ultrafast transfer of electron density along S-Ge bond. (b) Schematic depiction of the experimental geometry where sample orientation and linear polarization of an optical pump pulse are varied relative to the fixed polarization of the detected THz pulses. (c) Schematic diagram of the THz emission spectroscopy experiment.

Since BPVE relies on broken inversion symmetry, it is expected to occur not only in a monolayer, but generally in odd-layer monochalcogenide sheets.18, 22 In thicker, multi-layer films, we hypothesize that ferroelastic and corresponding ferroelectric polarization may occur in surface layers, resulting in shift currents that are confined to the surfaces. Ferroelectricity and ferroelasticity can even be present in the bulk phase of 2D GeS and related materials provided that inter-layer coupling is ferroelectric.16 The inherent 2D nature of these materials and the possibility of isolating or growing single or few-monolayer sheets provides other important advantages. Monochalcogenides with fewer layer numbers are expected to have a photoresponse that is significantly higher than that of the bulk, as the reduced dimensionality of a 2D material results in larger joint density of states above the band edge.23 Using 2D nanosheets with lateral dimensions on the order of 100 nm may also allow extracting a majority of carriers that contribute to photocurrent, and thus overcome the bottleneck in solar energy conversion efficiency due to the extremely short, 10-100 nm decay length of shift current carriers.6, 24 Here, we have used terahertz (THz) emission spectroscopy to detect the theoretically predicted shift currents in GeS nanosheets. Transient currents that vary on sub-picosecond time-scale 3 ACS Paragon Plus Environment

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result in emission of electromagnetic radiation in the THz frequency range. THz emission spectroscopy allows electrical contact-free, all-optical monitoring of transient currents unaffected by metal leads or other contacts by detecting THz radiation emitted by the ultrafast currents on the nanoscale. It has been used to observe currents due to an ultrafast laserinduced temperature gradient (Seebeck effect) in thermoelectric materials,25 photocurrents driven by the built-in surface depletion field, and photocurrents due to the difference in electron and hole mobilities (photo-Dember effect) in nanowires and nanorods.26-27 We have also recently used THz emission spectroscopy to detect ultrafast photocurrent in a macroscopically aligned single wall carbon nanotube film without an externally applied voltage, and ascribed its origin to top-bottom asymmetry in the morphology of the aligned arrays that creates a built-in electric field in the semiconducting nanotubes.28 THz emission spectroscopy has also been previously applied to probe shift currents in GaAs, CdS, CdSe, Bi2Se3 and other materials.3-5, 29-30 Shift current resulting from excitation with ultrafast pulses gives rise to THz emission that varies as  ∝  immediately above the surface of the photoexcited material.30-31

Figure 2. a) GeS is synthesized through the VLS route. (b)GeS has an orthorhombic crystal structure. (c-h) morphologies grown are flat nanoribbons and sheets with either the flat facet as (001) or (100). Most measurements here are presented on ribbons grown in the (001) direction. (i) XRD, (j) EDS, and (k) Raman characterization show predicted crystal structure and chemical composition.

GeS nanosheets (space group: Pnma) are synthesized through the vapor-liquid-solid method using established procedures and illustrated in Figure 2a.32 Fused silica or sapphire substrates are cleaned with acetone and then dried with high purity nitrogen. Clean substrates are coated with 20nm Au nanoparticles as a catalyst. Generally, 50mg of GeS powders (Sigma-Aldrich) are placed in the center of quartz tube furnace with substrates downstream. The quartz tube is evacuated to a base pressure of 10mtorr and flushed with high purity Argon gas three times. When the pressure is stable, 400°C is the center source temperature and 300°C is the substrate deposition temperature with 40-60sccm Ar flow rate and 2.5-5 torr for 1 hour. Synthesized GeS (pnma) nanomaterials (Figure 2(b)) are characterized with X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive x-ray (EDX), and Raman scattering, shown in Figure 2c-2k. X-ray Diffraction data was collected from a Bruker 4 ACS Paragon Plus Environment

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D8 Discover diffractometer with a copper source (Cu Kα1 = 1.5406Å, Kα2 = 1.54439Å). Scanning Electron microscope (SEM) images are collected from a 20keV Philips/FEI XL30 SFEG SEM. Transmission Electron Microscope (TEM) images, Energy Dispersive X-ray Spectroscopy (EDX) and selected area electron diffraction (SAED) were collected from a JEOL 2100F operating at 200keV with an Oxford EDX detector. For THz emission spectroscopy measurements, GeS nanosheets on a sapphire substrate were excited at normal incidence with 400 nm, 100 fs laser pulses from a 1 kHz amplified Ti:Sapphire source, as illustrated schematically in Figures 1b, 1c. The sample was placed behind a 1.5 mm diameter aperture (not shown) in the center of ~ 7mm diameter collimated excitation beam, so that the most uniform portion of the beam was used for excitation. A pair of off-axis parabolic mirrors focused the emitted THz pulses from GeS nanosheets within a 1.5 mm diameter spot on a sample onto a [110] ZnTe crystal where they were coherently detected by free-space electrooptic sampling.33 The wire-grid polarizer (Microtech Instruments; field extinction ratio of 0.01) ensured that only vertically-polarized component of the generated THz pulses was detected, and the polarization of the optical pump pulse (given by the angle of pump polarization relative to vertical, θpump) was varied by using a half-wave plate, as illustrated in Figure 1(b). Sample orientation could also be varied by rotating a sample by an angle θsample. As Figure 3a shows, photoexcitation of GeS nanosheets with 400 nm pulses with a fluence in the 1-15 µJ/cm2 results in emission of nearly single-cycle electromagnetic field transients. THz pulses presented in Figure 3a have been taken in different locations on the sample. Chosen excitation photon energy (3.1 eV) guarantees direct interband excitation, and emission of THz pulses indicates that the photoexcitation excites transient shift current.2 As  ∝  immediately above the nanosheet surface and again reimaged at the detector crystal,

detecting vertical component of emitted THz pulses in normal incidence geometry is sensitive exclusively to in-plane shift current in this specific direction. THz amplitude spectra corresponding to pulses in Figure 3a are shown in Figure 3b. THz generation in GeS nanosheets is efficient: optical-to-terahertz conversion efficiency, normalized to the thickness of the emitting crystal is higher in GeS nanosheets compared to (110) ZnTe with 800 nm excitation, which is a standard THz pulse source used for THz time-domain spectroscopy (Supplementary Figure S1). Note that the sapphire substrate with no GeS nanosheets does not emit any THz radiation, eliminating the possibility of substrate contribution to the observed effects (Supplementary Figure S2). Moreover, excitation with 15 µJ/cm2 fluence of 800 nm pulses (corresponding to the photon energy of 1.55 eV; see Supplementary Figure S2) also does not result in THz emission, underscoring that strong emission of THz radiation from unbiased GeS nanosheets excited at normal incidence results from above-band-gap photoexcitation rather than from optical rectification.

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Figure 3. (a) THz waveforms generated by photoexciting 1.5 mm diameter spots in three different locations on GeS nanosheet sample, and (b) - the corresponding amplitude spectra. For these, pump polarization was parallel to THz detection (θpump=0°). Solid black curve, offset for clarity, is the amplitude of a THz pulse emitted by a 1 mm-thick ZnTe crystal source and detected using an identical ZnTe crystal detector, and thus represents the response function of ZnTe detector. Dashed line shows a smoothed detector response function without the water absorption lines; c) Dynamics of shift current excited by the pump pulse with 100 fs duration in a material with hot carrier relaxation time varying from 20 fs to 1000 fs using a model proposed by Braun 30 et al. Inset in (b) shows the amplitude spectra corresponding to transients in (c). (d) Model shift current transients with a filter corresponding to ZnTe detector response function (dashed line in (b)) applied.

Temporal properties of the observed THz pulses can be analyzed using a model proposed by Braun et al.30 According to it, photoexcitation results in an instantaneous charge displacement x0H(t), where x0 is a spatial shift of electron density along S-Ge bond predicted to be ~ 0.52 Å,18 and H(t) is a unit step function. Transient shift current is then given by the convolution of the temporal derivative of this charge displacement with the pump intensity envelope Ip(t): 

 ∝  



 

 ∗  , where τsh is a phenomenological decay time that accounts for hot

carrier relaxation. Shift current transient is bipolar when the relaxation time is comparable to the pump duration. Figure 3c shows temporal profiles of the shift current for pump duration of 100 fs and decay time τsh ranging from 20 fs to 1000 fs. Duration of the transients is on the order of the excitation pulse duration (100 fs), and the corresponding frequency spectrum 6 ACS Paragon Plus Environment

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extends to ~ 10 THz (Figure 3d). The duration of the experimental THz transients (Figure 3a) is longer as ZnTe detector crystal acts as a band pass filter, essentially removing all frequencies below ~ 0.2 THz and above ~ 2.5 THz. Black curve in Figure 3b, offset for clarity, corresponds to the THz amplitude of a 1 mm-thick ZnTe crystal source detected using an identical ZnTe crystal detector, and thus represents the response function of ZnTe detector. Amplitude spectra corresponding to the THz pulses emitted by GeS nanosheets have the same spectral profile as ZnTe, suggesting that the observed THz transients do not accurately represent true ultrafast dynamics of shift current in GeS nanosheets. We have applied a smoothed detector response filter (dashed line in Figure 3b) to the model shift current transients so they can be compared to experimental THz waveforms. Resulting calculated transients are shown in Figure 3d, and they closely resemble the observed THz waveforms. Qualitative comparison of the bipolar THz pulses detected using optical rectification in ZnTe crystal with model shift current transients (Figure 3d) suggests that hot carrier relaxation time is comparable to pump duration of 100fs, likely falling in the range of 20 to sub-1000 fs. Decay time range of 50-1000 fs corresponds to decay length of 2-100 nm, in good agreement with earlier predictions for ferroelectric materials.6, 24 Analysis of the excitation power and polarization dependence of the shift current provide further information beyond the dynamics. Amplitude of the shift current that is responsible for BPVE is expected to be linear in incident intensity, and can be expressed as: ! =

#$% !(( ' ( ( −, 2

where ! is current in the a direction, (  is electric field of the optical excitation at frequency  polarized in the b direction, ' !(( is a photoresponsivity tensor, c is the speed of light, and $% is the permittivity of free space.10 In a GeS nanosheet, shift current flows in the plane of the sheet in the direction of displacement x0 (e.g, Ja=Jx), as illustrated in Figure 1a, resulting in a strongly polarized THz emission. Theory predicts that ' *** is large for above-gap excitation, while ' *++ is near zero.18 Thus, only the component of the pump-pulse electric field that is polarized along the specific nanosheet contributes to the shift current. A 1.5 mm field of view contains thousands of isolated, randomly oriented nanosheets which are excited simultaneously. Nanosheet dimensions are smaller than the wavelength of the emitted THz radiation and can be considered as point sources of polarized emission. Their emission is in phase, and thus electric fields emitted by separate nanosheets is additive at the detector. A non-zero cumulative electric field emitted by a large disordered ensemble of GeS nanosheets results from the overall weak anisotropy in the nanosheet alignment, whereas electric fields due to shift currents flowing in different directions in the plane of the sample do not cancel overall. In this situation, varying the excitation pulse polarization while keeping the sample orientation unchanged relative to THz detection direction (θsample=0°) only serves to alter the relative contributions of individual nanosheets. Figure 4 examines the pump polarization dependence of THz emission taken in a specific location (identified as location 3) on the sample 7 ACS Paragon Plus Environment

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(Figure 3a). Cumulative THz emission from a disordered nanosheet ensemble is only weakly dependent on the polarization of the excitation pulse. As the excitation polarization is rotated from vertical (parallel to the THz detection) to horizontal, the THz pulse peak is modulated by < 15% (Figure 4a), and the shape of the pulse remains unchanged regardless of excitation polarization (Figure 4b). A small variation of emission with pump polarization results from accidental overall alignment of nanosheets in this particular sample location. To confirm this hypothesis and demonstrate that the shift current magnitude direction and the corresponding amplitude and polarity of the emitted THz pulses are dictated primarily by the intrinsic polarization of GeS nanosheets, we have also explored how rotating the sample (and therefore every nanosheet contributes to the observed emission) while maintaining an unchanged pump polarization affects the resulting THz transient seen at the detector. Figure 5 shows how the THz pulse emitted by an ensemble of GeS nanosheets changes as all nanosheets are rotated in plane. Data presented in this figure has been taken in location 1 (Figure 3a), and similar data for another location is shown in Supplementary Figure S3. As expected, the peak polarization has a cosine dependence on the angle that the sample makes with respect to the detection direction. In this particular location on the sample, overall emission is strongest when the sample is aligned along the detection direction. For any two waveforms taken with the sample rotated by 180°, the polarity of the emission reverses while the amplitude shows only minimal change. Finally, Figure 6 shows that the electric field of THz pulses emitted by the GeS nanosheet ensemble is linearly dependent on absorbed fluence of 400 nm pulses, while the shape of THz waveforms is unchanged. Measurements presented in this figure were carried out with θpump=0° in location 2 on the sample (Fig. 3(a)), and absorbed fluence was calculated by subtracting reflected and transmitted fluence from incident one. THz pulse peak value is shown in the inset, and the red line is a line fit to experimental data (black squares). Linear fluence dependence of the emitted THz pulse electric field, and therefore, of transient current in the sample, confirms that GeS nanosheets exhibit shift current.

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Figure 4 (a) THz pulse peak as a function of linear polarization of the pump; inset – schematic depiction of the experimental geometry; (b) selected THz waveforms, corresponding to the minima and maxima in Figure 4a.

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Figure 5. Sample orientation dependence of THz generation by shift currents in GeS nanosheet ensemble. (a) Peak THz signal as a function of sample orientation. (b) THz waveforms taken at different sample orientations, indicating that rotating sample by 180° reverses polarity of the emitted pulse.

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Figure 6. THz generation in GeS nanosheets: THz waveforms at different absorbed photoexcitation fluence values. Inset: Peak electric field as a function of absorbed fluence; red line – linear fit.

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In summary, we have presented experimental evidence of shift current generation in GeS nanosheets in response to above-band-gap excitation supporting recent theoretical prediction.18 We find that photoexcitation with 400 nm pulses at normal incidence leads to emission of nearly single-cycle THz pulses without external bias voltage, indicating excitation of non-zero transient current in GeS nanosheet sample. Linear dependence of the generated THz pulse amplitude, and therefore the shift current, on excitation fluence confirms this hypothesis. We also find that the direction of the shift current, and the corresponding polarity of the emitted THz pulses is determined by the spontaneous polarization in the ferroelectric GeS nanosheets. Since the shift current is the dominant mechanism for BPVE, highly efficient shift current photoexcitation in GeS nanosheets and the bandgap significantly smaller than that in conventional ferroelectrics such as BiFeO3 suggest applications of these new layered materials in third generation photovoltaics. Theory predicts that GeS nanosheets can generate zero-bias photocurrents in response to excitation with photon energies ~ 1.9 eV and higher, covering most of the visible range. Experimental measurements with variable excitation wavelength would be necessary to confirm this. Reducing lateral dimensions of nanosheets to ~ 100 nm or less, comparable to shift carrier relaxation length, will be required to ensure optimal extraction of shift carriers. In addition to solar energy conversion, GeS nanosheets may soon also find uses as efficient, ultrathin THz sources.

Acknowledgements L.V.T. acknowledges start-up funds from Worcester Polytechnic Institute. K.J.K. acknowledges support from the Office of Naval Research N00014-16-1-3161-0. We acknowledge fruitful discussions with Dr. A. Zozulya, and thank him for a careful reading of the manuscript. We thank R. Steele and T. Partington for technical support. Supporting Information Available: Comparison of THz generation by GeS nanosheets and standard 1 mm thick [110] ZnTe crystal, demonstration of the lack of THz generation under below bandgap excitation and lack of THz generation by the substrates, and additional data on sample orientation dependence of THz generation.

REFERENCES (1) Nastos, F.; Sipe, J. E. Optical Rectification and Current Injection in Unbiased Semiconductors. Phys. Rev. B 2010, 82, 235204. (2) Nastos, F.; Sipe, J. E. Optical Rectification and Shift Currents in Gaas and Gap Response: Below and above the Band Gap. Phys. Rev. B 2006, 74, 035201.

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(3) Schleicher, J. M.; Harrel, S. M.; Schmuttenmaer, C. A. Effect of Spin-Polarized Electrons on Terahertz Emission from Photoexcited GaAs. J. Appl. Phys. 2009, 105, 113116. (4) Bieler, M.; Pierz, K.; Siegner, U.; Dawson, P. Shift Currents from Symmetry Reduction and Coulomb Effects in (110)-Orientated GaAs\Al0.3Ga0.7As Quantum Wells. Phys. Rev. B 2007, 76, 161304. (5) Bieler, M. THz Generation from Resonant Excitation of Semiconductor Nanostructures: Investigation of Second-Order Nonlinear Optical Effects. IEEE J. Sel. To. Quantum Electron. 2008, 14, 458-469. (6) Butler, K. T.; Frost, J. M.; Walsh, A. Ferroelectric Materials for Solar Energy Conversion: Photoferroics Revisited. Energy Environ. Sci. 2015, 8, 838-848. (7) Tan, L. Z.; Zheng, F.; Young, S. M.; Wang, F.; Liu, S.; Rappe, A. M. Shift Current Bulk Photovoltaic Effect in Polar Materials—Hybrid and Oxide Perovskites and Beyond. Npj Computational Materials 2016, 2, 16026. (8) Spanier, J. E.; Fridkin, V. M.; Rappe, A. M.; Akbashev, A. R.; Polemi, A.; Qi, Y.; Gu, Z.; Young, S. M.; Hawley, C. J.; Imbrenda, D.; et al. Power Conversion Efficiency Exceeding the Shockley–Queisser Limit in a Ferroelectric Insulator. Nat. Photon. 2016, 10, 611-616. (9) Wang, F.; Grinberg, I.; Jiang, L.; Young, S. M.; Davies, P. K.; Rappe, A. M. Materials Design of Visible-Light Ferroelectric Photovoltaics from First Principles. Ferroelectrics 2015, 483, 1-12. (10) Young, S. M.; Rappe, A. M. First Principles Calculation of the Shift Current Photovoltaic Effect in Ferroelectrics. Phys. Rev. Lett. 2012, 109, 116601. (11) Wang, F.; Rappe, A. M. First-Principles Calculation of the Bulk Photovoltaic Effect in Knbo3 and (K,Ba)(Ni,NbO3). Phys. Rev. B 2015, 91, 165124. (12) Ji, W.; Yao, K.; Liang, Y. C. Bulk Photovoltaic Effect at Visible Wavelength in Epitaxial Ferroelectric BiFeO3 Thin Films. Adv. Mater. 2010, 22, 1763-1766. (13) Grinberg, I.; West, D. V.; Torres, M.; Gou, G.; Stein, D. M.; Wu, L.; Chen, G.; Gallo, E. M.; Akbashev, A. R.; Davies, P. K.; et al. Perovskite Oxides for Visible-Light-Absorbing Ferroelectric and Photovoltaic Materials. Nature 2013, 503, 509-512. (14) Zheng, F.; Takenaka, H.; Wang, F.; Koocher, N. Z.; Rappe, A. M. First-Principles Calculation of the Bulk Photovoltaic Effect in CH3NH3PbI3 and CH3NH3PbI3–XClx. J. Phys. Chem. Lett. 2015, 6, 31-37. (15) Fei, R.; Li, W.; Li, J.; Yang, L. Giant Piezoelectricity of Monolayer Group Iv Monochalcogenides: Snse, Sns, Gese, and Ges. Appl. Phys. Lett. 2015, 107, 173104. (16) Wu, M.; Zeng, X. C. Intrinsic Ferroelasticity and/or Multiferroicity in Two-Dimensional Phosphorene and Phosphorene Analogues. Nano Lett. 2016, 16, 3236-3241. 13 ACS Paragon Plus Environment

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(17) Fei, R.; Kang, W.; Yang, L. Ferroelectricity and Phase Transitions in Monolayer Group-Iv Monochalcogenides. Phys. Rev. Lett. 2016, 117, 097601. (18) Cook, A. M.; B, M. F.; de Juan, F.; Coh, S.; Moore, J. E. Design Principles for Shift Current Photovoltaics. Nat. Commun. 2017, 8, 14176. (19) Gomes, L. C.; Carvalho, A. Phosphorene Analogues: Isoelectronic Two-Dimensional Group-Iv Monochalcogenides with Orthorhombic Structure. Phys. Rev. B 2015, 92, 085406 (20) Makinistian, L.; Albanesi, E. A. First-Principles Calculations of the Band Gap and Optical Properties of Germanium Sulfide. Phys. Rev. B 2006, 74, 045206. (21) Wu, M. H.; Zeng, X. C. Intrinsic Ferroelasticity and/or Multiferroicity in Two-Dimensional Phosphorene and Phosphorene Analogues. Nano Lett. 2016, 16, 3236-3241. (22) Xin, C.; Zheng, J. X.; Su, Y. T.; Li, S. K.; Zhang, B. K.; Feng, Y. C.; Pan, F. Few-Layer Tin Sulfide: A New Black-Phosphorus-Analogue 2d Material with a Sizeable Band Gap, Odd-Even Quantum Confinement Effect, and High Carrier Mobility. J. Phys. Chem. C 2016, 120, 2266322669. (23) Wang, F.; Young, S. M.; Zheng, F.; Grinberg, I.; Rappe, A. M. Substantial Bulk Photovoltaic Effect Enhancement Via Nanolayering. Nat. Commun. 2016, 7, 10419. (24) Zenkevich, A.; Matveyev, Y.; Maksimova, K.; Gaynutdinov, R.; Tolstikhina, A.; Fridkin, V. Giant Bulk Photovoltaic Effect in Thin Ferroelectric BaTiO3 Films. Phys. Rev. B 2014, 90, 161409. (25) Takahashi, K.; Kanno, T.; Sakai, A.; Tamaki, H.; Yamada, Y. Picosecond Thermoelectric Dynamics in Layered Cobaltite Thin Films Probed by Terahertz Emission Spectroscopy. Phys. Rev. B 2015, 92. (26) Trukhin, V. N.; Buyskikh, A. S.; Kaliteevskaya, N. A.; Bourauleuv, A. D.; Samoilov, L. L.; Samsonenko, Y. B.; Cirlin, G. E.; Kaliteevski, M. A.; Gallant, A. J. Terahertz Generation by Gaas Nanowires. Appl. Phys. Lett. 2013, 103, 072108. (27) Lee, W. J.; Ma, J. W.; Bae, J. M.; Jeong, K. S.; Cho, M. H.; Kang, C.; Wi, J. S. Strongly Enhanced THz Emission Caused by Localized Surface Charges in Semiconducting Germanium Nanowires. Sci. Rep. 2013, 3, 1984. (28) Titova, L. V.; Pint, C. L.; Zhang, Q.; Hauge, R. H.; Kono, J.; Hegmann, F. A. Generation of Terahertz Radiation by Optical Excitation of Aligned Carbon Nanotubes. Nano Lett. 2015, 15, 3267-3272. (29) Laman, N.; Bieler, M.; van Driel, H. M. Ultrafast Shift and Injection Currents Observed in Wurtzite Semiconductors Via Emitted Terahertz Radiation. J. Appl. Phys. 2005, 98, 103507.

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(30) Braun, L.; Mussler, G.; Hruban, A.; Konczykowski, M.; Schumann, T.; Wolf, M.; Münzenberg, M.; Perfetti, L.; Kampfrath, T. Ultrafast Photocurrents at the Surface of the ThreeDimensional Topological Insulator Bi2Se3. Nat. Commun. 2016, 7, 13259. (31) Priyadarshi, S.; Leidinger, M.; Pierz, K.; Racu, A. M.; Siegner, U.; Bieler, M.; Dawson, P. Terahertz Spectroscopy of Shift Currents Resulting from Asymmetric (110)-Oriented GaAs/AlGaAs Quantum Wells. Appl. Phys. Lett. 2009, 95, 151110. (32) Li, C.; Huang, L.; Snigdha, G. P.; Yu, Y.; Cao, L. Role of Boundary Layer Diffusion in Vapor Deposition Growth of Chalcogenide Nanosheets: The Case of Ges. ACS Nano 2012, 6, 88688877. (33) Titova, L. V.; Cocker, T. L.; Cooke, D. G.; Wang, X.; Meldrum, A.; Hegmann, F. A. Ultrafast Percolative Transport Dynamics in Silicon Nanocrystal Films. Phys. Rev. B 2011, 83, 085403.

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