Ultrafast Hot-Carrier Photovoltaics of Type-I ... - ACS Publications

Subscriber access provided by University of Newcastle, Australia

Letter

Ultrafast Hot-carrier Photovoltaics of Type-I Monolayer Heterojunctions in the Broad Spectral Ranges Ji Ho Sung, Soonyoung Cha, Hoseok Heo, Sangwan Sim, Juho Kim, Hyunyong Choi, and Moon-Ho Jo ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00846 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Photonics is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Ultrafast Hot-carrier Photovoltaics of Type-I Monolayer Heterojunctions in the Broad Spectral Ranges Ji Ho Sung,1,2,† Soonyoung Cha,1,4,† Hoseok Heo,1,2 Sangwan Sim,4 Juho Kim,1,3 Hyunyong Choi,4,* and Moon-Ho Jo,1,2,3,* 1

Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang 37673, Republic of Korea.

2

Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea.

3

Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea.

4

School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea

†These authors contributed equally to this work, *To whom correspondence should be addressed: [email protected] (H. Choi) and [email protected], (M.-H. Jo)

ABTRACT Strong interlayer photoresponses in monolayer (ML) semiconductor stacks, such as substantial light absorption and charge separation across interlayer band alignments, suggest potentials for two-dimensional photovoltaics (PVs). Here, we report an interlayer PV conversion in a type-I ML heterojunction by ultrafast interlayer transfer of photoexcited hot carriers in the broad spectral ranges. Specifically, low-energy photoexcitation on a stack of a narrow-bandgap (Eg) Bi2Te3 few-layer and a large-Eg MoS2 ML, permits interlayer transfer of transient hot carriers from the Bi2Te3 layer to the excitonic states of the neighbouring MoS2 ML within a time scale of ~ 70 fs, producing interlayer charge separation. Thereby we achieve substantial conversion efficiency from a MoS2 ML with visible to infrared light illumination.

ACS Paragon Plus Environment

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS: Two-dimensional materials, Photovoltaics, Transition-metal dichalcogenide, Hot carrier injection, Ultrafast charge transfer


Page 2 of 23

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Efficient light-to-electricity conversion in the broad spectral range is an important step forward in producing next-generation photovoltaics (PVs). 1 Transition-metal chalcogenides (TMChs) exhibit various electronic structures in the atomically thin regime,2,3,4,5,6,7,8,9,10,11,12,13,14 ranging from a vanishing energy bandgap (Eg) in topological insulators to distinct singularities near the band edges in large-Eg semiconductor monolayers (MLs).15,16 Those two-dimensional (2D) electronic structures provide diverse excitation and relaxation routes of photoexcited quasiparticles that are spectrally and kinetically distinctive. Such photoresponses can be manipulated in the artificial 2D stacks,17 where dissimilar TMCh MLs are bound to form new sets of electronic superstructures or band alignments.18,19,20,21,22,23,24 Previous studies reported that the type-II heterojunctions in large-Eg ML stacks induce charge separation across the band alignments by ultrafast interlayer transfer of photoexcited carriers that are generated by direct-Eg optical transitions in constituent MLs.25 However, in principle, interlayer charge separation can be also generated in an alternative route by low-energy photo-excitations in other types of the 2D band alignments in designer TMCh ML stacks. Here, we report a type-I 2D stack, composed of a large-Eg MoS2 ML and a narrow-Eg Bi2Te3 few-layers. Therein, the transient hot carriers in Bi2Te3 few-layers that are excited by the sub-Eg (of MoS2 ML) photons are transferred to the A and B excitonic states of the MoS2 ML on an ultrafast time scale of ~70 fs, to drive interlayer charge separation. As a result, the stack operates as a 2D excitonic PV cell in the broad excitation ranges down to near infrared. In our PV cells, the few-layer Bi2Te3 (or Sb2Te3) serves as an efficient hot carrier reservoir in the broad absorption range, which is qualitatively different from usual metals for hot carrier excitations; optically, it possesses a fairly large absorption crosssection from the near-infrared to the ultraviolet range, and electrically, it efficiently creates lowenergy hot carriers, whose transient time scale (~10 ps) is much slower than in normal metals of


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the ~ fs order.6,8,26,27,28,29 These long lived energetic carriers in Bi2Te3 (or Sb2Te3) are attributed to suppressed electron-phonon scattering process with its narrow Eg of ~ 160 meV, which is an order of magnitude larger than the highest phonon energy(~23 meV).30,31 The MoS2 ML serves as an efficient photon absorber in the visible range (a direct Eg of 1.89 eV), with the distinct excitonic states.5,14,32 The charge separation at the 2D heterointerfaces in our work is reminiscent of charge transfer excitons at the donor/acceptor heterointerfaces in organic heterojunctions.33 Although it is perceived that both heterojunction systems suffer from large exciton binding energy due to the poorly screened Coulomb potential,32,34,35 our strategy toward the spectral extension of PV conversion with ultrafast hot carrier transfer may suggest another pathway to achieve higher conversion efficiency as ultrathin planar PV platforms of large-areas.

MoS2 MLs and Bi2Te3 (or Sb2Te3) few-layers (7 nm in thickness or 7 quintuple layers) were separately crystallized by vapor transport synthesis in a 12-in hot-wall quartz tube.8,9,10,36,37 Vertical stacks of such 2D crystals were fabricated by a mechanical transfer method on SiO2/Si substrates (Figure 1a) – see also Figure S1. Multiple electrical contacts were made by e-beam lithography in such a way that one can selectively collect photoexcited carriers (photocurrent, Iph) within the individual layers or across the stack in a spectroscopic manner by local illumination with a supercontinuum laser.38,39 For example, we identified A and B excitonic transitions from the top MoS2 ML in the Iph spectra by local illumination at the MoS2 ML-metal contact area under the finite bias voltage (Vb), as in Figure S2, ensuring the crystal quality.9,40 We expected a type-I band alignment (the inset of Figure 1b) in the stack due to the fact that the work function of chemical vapor deposition grown few-layers Bi2Te3 (~4.3 eV) is larger than the electron affinity of MoS2 (~4.0 eV), 41 , 42 and found such a signature from the rectifying Idark-V


Page 4 of 23

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

characteristics (Figure 1b). This band offset can be electrically tuned by applying the Vg. The forward Idark of the stack is strongly enhanced with the positive Vg, due to the n-type semiconductor character of MoS2 ML. Of principal interest in this work is the interlayer charge separation upon light illumination (the PV responses), which can be spectrally captured with a pair of MoS2 and Bi2Te3 contacts. Figure 1c shows the Iph-V characteristics upon the local illumination of monochromatic light on the junction area at various excitation energies; at hν = 2.33 eV the PV responses are clearly marked by the distinct open circuit voltage (Voc) and short circuit current (Isc). We found that the PV response persists under the light illumination down to the near IR range of 1.17 eV, which is far below the Eg (1.89 eV) of MoS2 MLs. This indicates that the interlayer charge separation is efficiently achieved across the Bi2Te3-MoS2 junction in the broad absorption range. At hν < 1.89 eV, because the top MoS2 MLs are completely transparent, the charge separation is solely achieved by the low-energy (i.e., hν < Eg of MoS2 MLs) carrier excitation in the bottom Bi2Te3 layers, followed by interlayer transfer. That is, provided that the photoexcited electrons in Bi2Te3 layers efficiently transfer to MoS2 MLs, leaving holes behind, the spatial charge separation is established across the heterointerfaces (Figure 1d). Hereafter, we termed these photoexcited electrons in Bi2Te3 layers as hot electrons (carriers), because the excitation photon energy in this study is far greater than the Eg of Bi2Te3 (~160 meV). At hν > 1.89 eV, the photocarrier generation is additionally made by the interband optical transitions in the top MoS2 MLs, followed by the interlayer transfer of holes, to establish the interlayer charge separation. In both cases, we observed the similar magnitude of Voc (~ 0.2 eV), which is very close, but slightly larger than the Eg of Bi2Te3 layers. This observation is consistent with the PV principle that the Voc of the stack-type cells is fundamentally limited by the quasi-Fermi level difference between Bi2Te3 layers and MoS2 MLs.1 Qualitatively the same


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PV responses were observed in MoS2 MLs/Sb2Te3 few-layers stacks, which is another narrow-Eg semiconductor with the similar band alignment (Figure S2-4).

The observed interlayer charge transfer characteristics was investigated in more details by ultrafast optical pump and white-light probe spectroscopy (Figure 2).43,44 We choose the pumpphoton energy (hνpump = 0.99 eV) on MoS2/Bi2Te3 stacks to be far below the Eg of MoS2, such that the pump pulse generates hot carriers solely in the Bi2Te3 layers. Two sets of experiments were performed. In the first experiment (the upper panel of Figure 2a), we determined the transient absorption responses (∆T/T0 < 0, where ∆T is the pump-induced transmission change, and T0 is the transmission without the pump) of Bi2Te3 only (red dots) and the ∆T/T0 of MoS2/Bi2Te3 stacks (black dots), measured at the probe-photon energy hνprobe of 1.74 eV. Because hνprobe is also below (above) the Eg of MoS2 (Bi2Te3), one can expect that the photoresponses of Bi2Te3 layers should be different from those of MoS2/Bi2Te3 stacks.7,45 By comparing the decay dynamics of the transients in the two sets of samples, the effect of the photoexcited hot carrier transfer time from Bi2Te3 to MoS2 can be definitively determined. They show similar rising dynamics within the time-resolution of our probe pulse (~ 50 fs).7 While the Bi2Te3 transient shows a single-exponential decay of 6.5 ps, the MoS2/Bi2Te3 stacks exhibit an extremely fast τ1 of 70 fs with decreased differential absorption, where the transient can fit with fast (τ1) and slow (τ2) decay components in a bi-exponential function. Then, the transient relaxes with a slow τ2 of 6.5 ps. Here, we emphasize that this rapid absorption quenching (τ1 ≈ 70 fs) occurs only in the MoS2/Bi2Te3 stacks, and no such fast decay component was observed in the Bi2Te3 layers. The measured slow decay time is consistent with previous studies of slow cooling dynamics in the Bi2Te3 layers and fast decay time of ~70 fs is even faster than the time needed to


Page 6 of 23

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

recover Fermi-Dirac distribution in the Bi2Te3 layers.30,31 The fact that no pump-probe signal was observed in MoS2 MLs (blue dots in the lower panel of Figure 1a) further corroborates that the hot electron transfer takes place only in the MoS2/Bi2Te3 stacks. In the second experiment, this hot-carrier transfer was confirmed by scanning the probe-photon energy across the A and B exciton energies of MoS2 MLs. Figure 2b shows the spectrally resolved dynamics in the MoS2/Bi2Te3 stacks (black circle) as a function of the pump-probe delay. Under the same pump energy and fluence condition, the stack shows negative transmission changes (-∆T/T0) over the probe photon energies, i.e., increased absorption. In particular, we observed that the absorption changes in the stacks are significantly reduced at two specific probe energies, which are exactly matched with the A and B exciton energies of MoS2, that is, 1.89 and 2.1 eV, respectively.46 The dynamics of Bi2Te3 are spectrally featureless,47 regardless of the pump-probe delay (red circle), which can be regarded as the intralayer pump-induced hot carrier generation and cooling within the Bi2Te3 layers.26,48 Thus, our observations in the pump-probe measurements provide direct evidence of the charge separation, whose ultra-fast kinetics originates from the photoexcited hot electron transfer from the Bi2Te3 layers to the A and B excitonic population states of the MoS2 MLs (Figure 2c).

Figure 3a is the Iph spectrum (the red trace) collected with a pair of MoS2 and Bi2Te3 contacts under zero-bias voltage (Vb), thus representing the spectral Isc of the stacks – see also Figure S2 for the MoS2 and Sb2Te3 stacks, which shows essentially the same spectral responses. We clearly verified that the PV Isc persist down to the near IR range of 0.75 eV, comparing with the Iph spectra collected by local illumination at the metal-MoS2 ML contacts (thus representing the metal junction photoresponses of the MoS2 ML alone). This observation is consistent with the


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

suggested interlayer charge separation mechanism of Figure 1c and d. The interlayer PV responses at the heterointerfaces are also examined by the spatial mapping of the Isc in the stack (Figure 3b-d) by scanning a focused laser – see also Figure S3. We immediately identified that the Isc generation is localized in the stack region over the spectral range, suggesting the interlayer charge separation is the key element to the PV conversion, although the Isc intensity is not homogeneously distributed over the stack region, presumably due to the non-uniform contact of the two layers during the manual transfer.49

Earlier reports on monolayer semiconductor PV cells are typically based on either intralayer (lateral) or interlayer (vertical) p-n junctions.19-25 By contrast, the heterojunction cells operating by the hot carrier injection across the type-I van der Waals interfaces has been rarely addressed. We discuss the light-to-power conversion of our stacks (of the effective junction area of 50 µm2) in comparison with earlier works of ML p-n junctions. Figure 4a is the PV I-Vb characteristics under various light illumination intensities at hν = 2.33 eV. The inset is the generated power (Ppv), which is quadratic to the applied Vb. We measure the maximum power to be over 600 pW from the MoS2/Bi2Te3 stacks (and 5,000 pW from the MoS2/Sb2Te3 stacks in Figure S4) under 170 µW light illumination. Typically, the generated power is ranged from 100 pW to 1 nW from more than 10 devices, and is comparable to those of the earlier MoS2 intra-ML p-n PV cells.50,51,52 Considering the measured power loss due to the lateral charge transport in our stacks, it suggests that the charge separation is reasonably efficient by hot carrier transfer in the stack. Remarkably, such PV responses persist down to hν = 1.17 eV, as in Figure 4c, with a Ppv of 30 pW under 880 µW light excitation. Last, we report the external quantum efficiency, ηext = (Iph/e)/(Popt/hv), when Vb = 0, Iph ~ Isc. Figure 4b and d are such EQEs for the MoS2/Bi2Te3 stack upon light illumination


Page 8 of 23

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

above and below the Eg of MoS2 MLs, which are electrically tuneable by the Vg. The maximum EQE was measured to be ~ 10-4 at hν = 2.33 eV. It also persists at hν = 1.17 eV, albeit with values lower by two orders of magnitude. Note that these EQE values can be converted to photon conversion efficiency (PCE), defined as = ( ∙ ℎ )⁄( ∙ ), of 20 % at the excitation of 2.33 eV and 0.2 % for 1.17 eV, where Jph is the photocurrent density, h is the Planck constant, c is the speed of light, Ihv is the areal light intensity, λ is the wavelength, and e is the elementary charge, Rather poor quantum efficiency of our device is due to lateral charge collection, and it can be overcome by vertical charge collection scheme made with MoS2/Bi2Te3 vertical stack between two graphene layers.25 Recent experimental finding suggests clever way of extending lifetime (~50 ps) of excited carriers in topological insulators by photoinduced electron-hole asymmetry which can be used for high-efficiency 2D hot-carrier photovoltaic cells.53

METHODS Materials: Vapour Transport Synthesis of 2D Bi2Te3 and Sb2Te3 crystals 2D Bi2Te3 and Sb2Te3 crystals of few QLs were grown by vapour transport synthesis from solid powder precursors. High-purity Bi2Te3 or Sb2Te3 powders were placed in alumina boats and loaded in the centre of a 12-in. hot-walled quartz-tube furnace. SiO2 (300 nm)/Si wafers as the growth substrates were also placed 10 cm downstream from the tube centre, considering the temperature gradient along the tube furnace. Prior to heating, the furnace was evacuated to 10−3 Torr and purged with high-purity nitrogen (N2) for 30 min to eliminate residual oxygen. Then, the furnace was heated to 530 ºC (for Sb2Te3) or 540 ºC (for Bi2Te3) for 30 min under 5 sccm of high-purity argon (Ar) flow as a carrier gas under a total pressure of 0.1 Torr.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

Spatially and spectrally resolved photocurrent spectroscopy For the spectral photocurrent measurements, we used a broadband supercontinuum laser (450 nm ≤ λ ≤ 2000 nm) combined with a monochromator for the high-resolution spectra. During the wavelength scanning, the photocurrent was measured by a lock-in technique with a chopper frequency of 500Hz and subsequently normalized to the photon flux. The chopped laser beam was focused by a microscopic lens (NA = 0.8) and illuminated the ML channel region of the devices. Ultrafast pump-probe spectroscopy To perform the ultrafast pump-probe spectroscopy, femtosecond pulses at photon energy of 1.55 eV were generated by a 250 kHz Ti:sapphire regenerative system (Coherent RegA9050). The laser output pulse was separated into two pulses by a ratio of 7:3. The 70% laser pulse was injected into an optical parametric amplifier (Coherent OPA 9850) to generate infrared pump pulses at a photon energy of 0.99 eV. The other was focused into a 1 mm-thick sapphire disk to create a white-light supercontinuum source, which served as a probe pulse. To suppress the scattering-induced artefact, the pump and probe pulses were focused on the sample with noncollinear geometry. The spot size of our pump and probe beams were 100 µm and 50 µm, respectively. The spectrally-resolved transmission of probe pulses was obtained by Si photodetector and monochromator (Newport 74125), and the pump-induced transient differential signal was recorded using a lock-in amplifier (SR850) with a chopping frequency of 10 kHz. During measurements, the pump-probe delay was controlled using a computer-controlled mechanical delay stage (Newport M-IMS300LM).


Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

ACKNOWLEDGEMENT This work was supported by the Institute for Basic Science (IBS), Korea under the Project Code number IBS-R014-G1-2016-a00. S. Sim, and H. Choi were supported by National Research Foundation of Korea (NRF) through the government of Korea (MSIP) (NRF-2011-0013255).

Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

Figure 1. (a) Schematic diagram and an optical image of the vertically stacked MoS2/Bi2Te3 heterointerface. Scale bar is 5 m. (b) Dark I-V curves of the MoS2/Bi2Te3 device at various back gate voltages measured across the heterojunction with a pair of MoS2 and Bi2Te3 contacts. Inset: Energy band alignment (type-I alignment) of the heterojunction. (c) Photoresponse characteristics under 2.33 eV (left y-axis), 1.38 eV, and 1.17 eV (right y-axis) light illumination at the heterointerface of the MoS2/Bi2Te3 vertical stack device. (d) Band alignments and charge separation at the MoS2/Bi2Te3 heterointerface. Upon light illumination of hν < 1.89 eV (the Eg of MoS2 ML), interlayer charge transfer is attributed solely to the sub-Eg excited hot carrier transfer across the two layers. At hν > 1.89 eV light illumination, interlayer charge separation is dominated by the intralayer interband transitions of the MoS2 ML followed by charge transfer.

Figure 2. (a) Time-resolved ultrafast transmission changes (-∆T/T0) in the MoS2/Bi2Te3 stacks (black), Bi2Te3 layers (red), and MoS2 MLs (blue). The measurements were performed at room temperature with hνprobe of 1.74 eV (below the Eg of MoS2 ML) under a pump fluence of 970 µJ/cm2. The solid lines (red and black) are the exponential fits multiplied by a Heaviside step function. (b) Spectrally resolved -∆T/T0 spectra of the MoS2/Bi2Te3 heterojunction (black) and Bi2Te3 layers (red) after 0.99eV optical excitation. Note that the transient spectra in the MoS2/Bi2Te3 heterojunctions exhibit pronounced dips at A and B excitonic resonances (dashed lines), which are direct evidence of interlayer charge transfer from the Bi2Te3 layers to the MoS2 MLs. (c) Illustration for hot carrier transfer from Bi2Te3 layers to A and B excitonic states of the MoS2 ML in the pair momentum space.

Figure 3. (a) Optical power normalized local Iph spectra of a MoS2/Bi2Te3 heterojunction device


Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

under zero-bias voltage with the pair of MoS2 and Bi2Te3 contacts (red line) and two MoS2 contacts (blue line). Iph under zero-bias voltage between two MoS2 contacts is generated in metal-MoS2 junction area. (b) Optical image of the MoS2/Bi2Te3 stack device. (c-d) Scanning Isc images of the MoS2/Bi2Te3 heterojunction device under (c) 2.33 eV and (d) 0.94 eV light illumination. The Isc is generated on the entire area of MoS2/Bi2Te3 heterojunction which shows that Isc by vertical interlayer transport dominates, rather than by the lateral transport.

Figure 4. (a,c) I-V curves under (a) 2.33 eV and (c) 1.17 eV optical excitation of the MoS2/Bi2Te3 stacks with various laser powers. Inset: Generated PV power (Ppv = IdsVds) as a function of Vb for different excitation optical powers. (b,d) External quantum efficiency under (b) 2.33 eV excitation for Vb = 0 at various gate voltages and (d) under 1.17 eV excitation.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

REFERENCES

1

Shockley, W.; Queisser, H.J. Detailed balance limit of efficiency of p-n junction solar cells, J. Appl. Phys. 1961, 32, 510-519. 2

Graham, M. W.; Shi, S.-F.; Ralph, D.C.; Park, J.; McEuen, P.L. Photocurrent measurements of supercollision cooling in graphene. Nature Phys. 2013, 9, 103-108. 3

Song, J. C. W.; Rudner, M. S.; Marcus, C. M.; Levitov, L. S. Hot carrier transport and photocurrent response in graphene. Nano Lett. 2011, 11, 4688-4692. 4

Tielrooij, K. J.; Song, J. C. W.; Jensen, S. A.; Centeno, A.; Pesquera, A.; Zurutuza Elorza, A.; Bonn, M.; Levitov, L. S.; Koppens, F. H. L.; Photoexcitation cascade and multiple hot-carrier generation in graphene. Nature Phys. 2013, 9, 248-252. 5

Polini, M.; Guinea, F.; Lewenstein, M.; Manoharan, H. C.; Pellegrini, V. Artificial honeycomb lattices for electrons, atoms and photons. Nature Nanotech. 2013, 8, 625-633. 6

Gabor, N. M.; Song, J. C. W.; Ma, Q.; Nair, N. L.; Taychatanapat, T.; Watanabe, K.; Taniguchi, T.; Levitov, L. S.; Jarillo-Herrero, P. Hot carrier-assisted intrinsic photoresponse in graphene. Science 2011, 334, 648-652. 7

Hong, X.; Kim, J.; Shi, S.-F.; Zhang, Y.; Jin, C.; Sun, Y.; Tongay, S.; Wu, J.; Zhang, Y.; Wang, F. Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures. Nature Nanotech. 2014, 9, 682-686. 8

Sung, J. H.; Heo, H.; Hwang, I.; Lim, M.; Lee, D.; Kang, K.; Choi, H. C.; Park, J.-H.; Jhi, S.H.; Jo, M.-H. Atomic Layer-by-Layer Thermoelectric Conversion in Topological lnsulator Bismuth/Antimony Tellurides, Nano Lett. 2014, 14, 4030-4035. 9

Heo, H.; Sung, J. H.; Jin, G.; Ahn, J.-H.; Kim, K.; Lee, M.-J.; Cha, S.; Choi, H.; Jo, M.-H. Rotation-misfit-free heteroepitaxial stacking and stitching growth of hexagonal transition-metal dichalcogenide monolayers by nucleation kinetics controls, Adv. Mater. 2015, 27, 3803-3810.

10

Heo, H.; Sung, J. H.; Cha, S.; Jang, B.-G.; Kim, J.-Y.; Jin, G.; Lee, D.; Ahn, J.-H.; Lee, M.-J.; Shim, J. H.; Choi, H.; Jo, M.-H. Interlayer orientation dependent light absorption and emission in monolayer semiconductor stacks. Nat. Comm. 2015, 6, 7372-7378.

11

Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically thin MoS2: A new direct-gap semiconductor. Phys. Rev. Lett. 2010, 105, 136805.

12

Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 transistors. Nature Nanotech. 2011, 6, 147-150.


Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

13

Radisavljevic, B.; Kis, A. Mobility engineering and a metal-insulator transition in monolayer MoS2. Nature Mater. 2013, 12, 815-820.

14

Komsa, H.; Krasheninnikov, A. V. Electronic structures and optical properties of realistic transition metal dichalcogenide heterostructures from first principles. Phys. Rev. B. 2013, 88, 085318. 15

Britnell, L.; Ribeiro, R. M.; Eckmann, A.; Jalil, R.; Belle, B. D.; Mishchenko, A.; Kim, Y.-J.; Gorbachev, R. V.; Georgiou, T.; Morozov, S. V.; Grigorenko, A. N.; Geim, A. K.; Casiraghi, C.; Castro Neto, A. H.; Novoselov, K. S. Strong light-matter interactions in heterostructures of atomically thin films. Science 2013, 340, 1311-1314. 16

Kośmider, K.; Fernández-Rossier, J. Electronic properties of the MoS2-WS2 heterojunction. Phys. Rev. B. 2013, 87, 075451. 17

Capasso, F. Band-gap engineering: from physics and materials to new semiconductor devices. Science 1987, 235, 172-176.

18

Terrones, H.; Lopez-Urias, F.; Terrones, M. Novel hetero-layered materials with tunable direct band gaps by sandwiching different metal disulfides and diselenides. Sci. Rep. 2013, 3, 1549. 19

Bernardi, M.; Palummo, M.; Grossman, J. C. Extraordinary sunlight absorption and one nanometer thick PVs using two-dimensional monolayer materials. Nano Lett. 2013, 13, 36643670. 20

Huang, C.; Wu, S.; Sanchez, A. M.; Peters, J. J. P.; Beanland, R.; Ross, J. S.; Rivera, P.; Yao, W.; Cobden, D. H.; Xu, X. Lateral heterojunctions with monolayer MoSe2-WSe2 semiconductors. Nature Mater. 2014, 13, 1096-1101. 21

Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.; Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.; Zhou, W.; Ajayan, P. M. Vertical and in-plane heterostructures from WS2/MoS2 monolayers. Nature Mater. 2014, 13, 1135-1142. 22

Duan, X.; Duan, X.; Wang, C.; Shaw, J. C.; Cheng, R.; Chen, Y.; Li, H.; Wu, X.; Tang, Y.; Zhang, Q.; Pan, A.; Jiang, J.; Yu, R.; Huang, Y.; Duan, X. Lateral epitaxial growth of twodimensional layered semiconductor heterojunctions. Nature Nanotech. 2014, 9, 1024-1030. 23

Fang, H.; Battaglia, C.; Carraro, C.; Nemsak, S.; Ozdol, B.; Kang, J. S.; Bechtel, H. A.; Desai, S. B.; Kronast, F.; Unal, A. A.; Conti, G.; Conlon, C.; Palsson, G. K.; Martin, M. C.; Minor, A. M.; Fadley, C. S.; Yablonovitch, E.; Maboudian, R.; Javey, A. Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides. Proc. Natl. Acad. Sci. USA. 2014, 111, 6198-6202.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

24

Yu, W. J.; Liu, Y.; Zhou, H.; Yin, A.; Li, Z.; Huang, Y.; Duan, X. Highly efficient gate-tunable photocurrent generation in vertical heterostructures of layered materials. Nature Nanotech. 2013, 8, 952-958. 25

Lee, C.-H.; Lee, G.-H.; van der Zande, A. M.; Chen, W.; Li, Y.; Han, M.; Cui, X.; Arefe, G.; Nuckolls, C.; Heinz, T. F.; Guo, J.; Hone, J.; Kim, P. Atomically thin p-n junctions with van der Waals heterointerfaces. Nature Nanotech. 2014, 9, 676-681. 26

Spicer, W.E. Photoemissive, Photoconductive, and Optical Absorption Studies of AlkaliAntimony Compounds, Phys. Rev. 1958, 112, 114-122.

27

Tsen, A.W.; Donev, L. A. K.; Kurt, H.; Herman, L. H.; Park, J. Imaging the electrical conductance of individual carbon nanotubes with photothermal current microscopy, Nature Nanotech. 2009, 4, 108-113. 28

Barkelid, M.; Zwiller, V. Photocurrent generation in semiconducting and metallic carbon nanotubes, Nature Photon. 2014, 8, 47-51. 29

Buscema, M.; Barkelid, M.; Zwiller, V.; van der Zant, H. S. J.; Steele, G. A.; CastellanosGomez, A. Large and tunable photothermoelectric effect in single-layer MoS2. Nano Lett. 2013, 13, 358-363.

30

Sobota, J. A.; Yang, S.; Analytis, J. G.; Chen, Y. L.; Fisher, I. R.; Kirchmann, P. S.; Shen, Z.-X. Ultrafast optical excitation of a persistent surface-state population in the topological insulator Bi2Se3. Phys. Rev. Lett. 2012, 108, 117403. 31

Hajlaoui, M.; Papalazarou, E.; Mauchain, J.; Lantz, G.; Moisan, N.; Boschetto, D.; Jiang, Z.; Miotkowski, I; Chen, Y. P.; Taleb-Ibrahimi, A.; Perfetti, L.; Marsi, M. Ultrafast surface carrier dynamics in the topological insulator Bi2Te3. Nano Lett. 2012, 12, 3532-3536.

32

Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.-Y.; Galli, G.; Wang, F. Emerging photoluminescence in monolayer MoS2. Nano Lett. 2010, 10, 1271-1275. 33

Zhu, X.; Monahan, N. R.; Gong, Z.; Zhu, H.; Williams, K. W.; Nelson, C. A. Charge transfer excitons at van der Waals interfaces. J. Am. Chem. Soc. 2015, 137, 8313-8320.

34

Tabachnyk, M.; Ehrler, B.; Gelinas, S.; Bohm, M. L.; Walker, B. J.; Musselman, K. P.; Greenham, N. C.; Friend, R. H.; Rao, A. Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals. Nature Mater. 2014, 13, 1033-1038. 35

Komsa, H.; Kransheninnikov, A. V. Effects of confinement and environment on the electronic structure and exciton binding energy of MoS2 from first principles. Phys. Rev. B. 2012, 86, 241201.


Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

36

Heo, H.; Sung, J. H.; Ahn, J.-H.; Ghahari, F.; Taniguchi, T.; Watanabe, K.; Kim, P.; Jo, M.-H. Frank–van der Merwe Growth versus Volmer–Weber Growth in Successive Stacking of a FewLayer Bi2Te3/Sb2Te3 by van der Waals Heteroepitaxy: The Critical Roles of Finite LatticeMismatch with Seed Substrates. Adv. Elect. Mater. 2017, DOI : 10.1002/aelm.201600375. 37

Ahn, J.-H.; Lee, M.-J.; Heo, H.; Sung, J. H.; Kim, K.; Hwang, H.; M.-H. Jo. Deterministic two-dimensional polymorphism growth of hexagonal n-type SnS2 and orthorhombic p-type SnS crystals. Nano Lett. 2015, 15, 3703-3708. 38

Kim, C.-J.; Lee, H.-S.; Cho, Y.-J.; Kang, K.; Jo, M.-H. Diameter-dependent internal gain in ohmic Ge nanowire photodetectors. Nano Lett. 2010, 10, 2043-2048. 39

Kim, C.-J.; Lee, H.-S.; Cho, Y.-J.; Yang, J.-E.; Lee, R. R.; Lee, J. K.; Jo, M.-H. On-nanowire band-graded Si:Ge photodetectors. Adv. Mater. 2011, 23, 1025-1029. 40

Mak, K. F.; He, K.; Shan, J.; Heinz, T. F. Control of valley polarization in monolayer MoS2 by optical helicity. Nature Nanotech. 2012, 7, 494-498.

41

Hao, G.; Qi, X.; Xue, L.; Cai, C.; Li, J.; Wei, X.; Zhong, J. Fermi level tuning of topological insulator Bi2(SexTe1-x)3 nanoplates. J. Appl. Phys. 2013, 113, 024306. 42

Lee, K.; Kim, H.-Y.; Lotya, M.; Coleman, J. N.; Kim, G.-T.; Duesberg, G. S. Electrical characteristics of molybdenum disulfide flakes produced by liquid exfoliation. Adv. Mater. 2011, 23, 4178-4182. 43

Sim, S.; Lee, D.; Noh, M.; Cha, S.; Soh, C. H.; Sung, J. H.; Jo, M.-H. Selectively tunable optical Stark effect of anisotropic excitons in atomically thin ReS2. Nature Comm. 2016, 7, 13569. 44

Cha, S.; Sung, J. H.; Sim, S.; Park, J.; Heo, H.; Jo, M.-H.; Choi. H. 1s intraexcitonic dynamics in monolayer MoS2 probed by ultrafast mid-infrared spectroscopy. Nature Comm. 2016, 7, 10768.

45

He, J.; Kumar, N.; Bellus, M. Z.; Chiu, H.-Y.; He, D.; Wang, Y.; Zhao, H. Electron transfer and coupling in graphene-tungsten disulfide van der Waals heterostructures. Nature Comm. 2014, 5, 5622.

46

Pogna, E. A. A.; Marsili, M.; Fazio, D. D.; Conte, S. D.; Manzoni, C.; Sangalli, D.; Yoon, D.; Lombardo, A.; Ferrari, A. C.; Marini, A.; Cerullo, G.; Prezzi, D. Photo-induced bandgap renormalization governs the ultrafast response of single-layer MoS2. ACS Nano 2016, 10, 11821188. 47

Jia, L.; Ma, W.; Zhang, X.; Ultrafast carrier dynamics in polycrystalline bismuth telluride nanofilm. Appl. Phys. Lett. 2014, 104, 241911.


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

48

It is indeed a rather complex process due to a nontrivial band structures of Bi2(Sb2)Te3 few MLs, as another subject of our future study, yet to be precisely determined including both carrier scattering and phonon scattering with/without topological surface states. At the moment, it is beyond to this study. 49

Haigh, S. J.; Gholinia, A.; Jalil, R.; Romani, S.; Britnell, L.; Elias, D. C.; Novoselov, K. S.; Ponomarenko, L. A.; Geim, A. K.; Gorbachev, R. Cross-sectional imaging of individual layers and buried interfaces of graphene-based heterostructures and superlattices. Nature Mater. 2012, 11, 764-767. 50

Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive photodetectors based on monolayer MoS2. Nature Nanotech. 2013, 8, 497-501. 51

Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y.; Jarillo-Herrero, P. Optoelectronic devices based on electrically tunable p–n diodes in a monolayer dichalcogenide. Nature Nanotech. 2014, 9, 262-267. 52

Pospischil, A.; Furchi, M. M.; Mueller, T. Solar-energy conversion and light emission in an atomic monolayer p-n diode. Nature Nanotech. 2014, 9, 257-261.

53

Hajlaoui, M.; Papalazarou, E.; Mauchain, J.; Perfetti, L.; Taleb-Ibrahimi, A.; Navarin, F.; Monteverde, M.; Auban-Senzier, P.; Pasquier, C. R.; Moisan, N.; Boschetto, D.; Meupane, M.; Hasan, M. Z.; Durakiewicz, T.; Jiang, Z.; Xu, Y.; Miotkowski, I.; Chen, Y. P.; Jia, S.; Ji, H. W.; Cava, R. J.; Marsi, M. Tuning a schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry. Nature Comm. 2014, 5, 3003.


Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Table of contents graphic


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a) Schematic diagram and an optical image of the vertically stacked MoS2/Bi2Te3 heterointerface. Scale bar is 5 µm. (b) Dark I-V curves of the MoS2/Bi2Te3 device at various back gate voltages measured across the heterojunction with a pair of MoS2 and Bi2Te3 contacts. Inset: Energy band alignment (type-I alignment) of the heterojunction. (c) Photoresponse characteristics under 2.33 eV (left y-axis), 1.38 eV, and 1.17 eV (right y-axis) light illumination at the heterointerface of the MoS2/Bi2Te3 vertical stack device. (d) Band alignments and charge separation at the MoS2/Bi2Te3 heterointerface. Upon light illumination of hν < 1.89 eV (the Eg of MoS2 ML), interlayer charge transfer is attributed solely to the sub-Eg excited hot carrier transfer across the two layers. At hν > 1.89 eV light illumination, interlayer charge separation is dominated by the intralayer interband transitions of the MoS2 ML followed by charge transfer. 170x45mm (300 x 300 DPI)


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 2. (a) Time-resolved ultrafast transmission changes (-∆T/T0) in the MoS2/Bi2Te3 stacks (black), Bi2Te3 layers (red), and MoS2 MLs (blue). The measurements were performed at room temperature with hνprobe of 1.74 eV (below the Eg of MoS2 ML) under a pump fluence of 970 µJ/cm2. The solid lines (red and black) are the exponential fits multiplied by a Heaviside step function. (b) Spectrally resolved -∆T/T0 spectra of the MoS2/Bi2Te3 heterojunction (black) and Bi2Te3 layers (red) after 0.99eV optical excitation. Note that the transient spectra in the MoS2/Bi2Te3 heterojunctions exhibit pronounced dips at A and B excitonic resonances (dashed lines), which are direct evidence of interlayer charge transfer from the Bi2Te3 layers to the MoS2 MLs. (c) Illustration for hot carrier transfer from Bi2Te3 layers to A and B excitonic states of the MoS2 ML in the pair momentum space. 170x60mm (300 x 300 DPI)


ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. (a) Optical power normalized local Iph spectra of a MoS2/Bi2Te3 heterojunction device under zerobias voltage with the pair of MoS2 and Bi2Te3 contacts (red line) and two MoS2 contacts (blue line). Iph under zero-bias voltage between two MoS2 contacts is generated in metal-MoS2 junction area. (b) Optical image of the MoS2/Bi2Te3 stack device. (c-d) Scanning Isc images of the MoS2/Bi2Te3 heterojunction device under (c) 2.33 eV and (d) 0.94 eV light illumination. The Isc is generated on the entire area of MoS2/Bi2Te3 heterojunction which shows that Isc by vertical interlayer transport dominates, rather than by the lateral transport. 170x51mm (300 x 300 DPI)


Page 22 of 23

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 4. (a,c) I-V curves under (a) 2.33 eV and (c) 1.17 eV optical excitation of the MoS2/Bi2Te3 stacks with various laser powers. Inset: Generated PV power (Ppv = IdsVds) as a function of Vb for different excitation optical powers. (b,d) External quantum efficiency under (b) 2.33 eV excitation for Vb = 0 at various gate voltages and (d) under 1.17 eV excitation. 170x57mm (300 x 300 DPI)


Ultrafast Hot-Carrier Photovoltaics of Type-I ... - ACS Publications

Recommend Documents