Limits...
Photothermoelectric and photovoltaic effects both present in MoS2.

Zhang Y, Li H, Wang L, Wang H, Xie X, Zhang SL, Liu R, Qiu ZJ - Sci Rep (2015)

Bottom Line: The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE).In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime.Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.

View Article: PubMed Central - PubMed

Affiliation: 1] State Key Laboratory of ASIC &System, School of Information Science and Technology, Fudan University, Shanghai 200433, China [2] State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem &Information Technology, Chinese Academy of Sciences, Changning Road 865, Shanghai 200050, China.

ABSTRACT
As a finite-energy-bandgap alternative to graphene, semiconducting molybdenum disulfide (MoS2) has recently attracted extensive interest for energy and sensor applications. In particular for broad-spectral photodetectors, multilayer MoS2 is more appealing than its monolayer counterpart. However, little is understood regarding the physics underlying the photoresponse of multilayer MoS2. Here, we employ scanning photocurrent microscopy to identify the nature of photocurrent generated in multilayer MoS2 transistors. The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE). In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime. Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.

No MeSH data available.


Related in: MedlinePlus

Photocurrent and Raman images of MoS2 transistor.(a) Short-circuit scanning photocurrent images taken at Vg = 15 V with a laser power of 100 μW. (b) Raman mapping image with the normalized E12g peak intensity of the MoS2 multilayer. In panels (a) and (b), the dashed lines indicate the edges of the source and drain electrodes while the white lines mark the boundaries of MoS2. (c) Energy-band diagram of a multilayer- MoS2 transistor under light illumination in the accumulation (Vg > 0 V) and depletion (Vg < 0 V) regimes. Electron-hole pairs are generated in the space-charge region by light absorption and contribute to PVE and PTE photocurrents, which are indicated by blue and red arrows, respectively. (d) Gate-dependent Seebeck coefficient (S, black line) and the open-circuit photovoltage (VPH, red line) with a 100-μW laser illuminated at the contact edge. The vertical blue dotted lines separate the regimes of accumulation, depletion and inversion while the horizontal black dashed line indicates the border between the electron and hole conductions.
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f3: Photocurrent and Raman images of MoS2 transistor.(a) Short-circuit scanning photocurrent images taken at Vg = 15 V with a laser power of 100 μW. (b) Raman mapping image with the normalized E12g peak intensity of the MoS2 multilayer. In panels (a) and (b), the dashed lines indicate the edges of the source and drain electrodes while the white lines mark the boundaries of MoS2. (c) Energy-band diagram of a multilayer- MoS2 transistor under light illumination in the accumulation (Vg > 0 V) and depletion (Vg < 0 V) regimes. Electron-hole pairs are generated in the space-charge region by light absorption and contribute to PVE and PTE photocurrents, which are indicated by blue and red arrows, respectively. (d) Gate-dependent Seebeck coefficient (S, black line) and the open-circuit photovoltage (VPH, red line) with a 100-μW laser illuminated at the contact edge. The vertical blue dotted lines separate the regimes of accumulation, depletion and inversion while the horizontal black dashed line indicates the border between the electron and hole conductions.

Mentions: To identify the mechanism of photocurrent generation, SPCM was conducted at zero source-drain bias (Vd = 0 V) with a well-focused laser beam scanning over the device area. The short-circuit photocurrent and Raman spectra were recorded simultaneously as a function of laser position. To avoid possible photo-damage to the MoS2 during experiment35, the step size was set to be 1 μm. Figure 3a shows the photocurrent image measured at Vg = 15 V, where two photocurrent extrema of opposite polarities are observed at the electrode edges. As expected from conventional PVE, the opposite built-in electric fields from space charges at the source/drain contacts create an asymmetrical photocurrent characteristic. Such PVE-induced photocurrents localized near electrodes have been widely reported in devices based on silicon nanowires36 and carbon nanotubes3738. However, it is remarkable to observe a significant photocurrent when the laser spot is centered inside the electrodes. The distance from the electrode edge is ~10 times larger than the laser spot size in order to make sure that the MoS2 channel is not illuminated. Besides, the light penetration depth is smaller than the thickness for the metal electrodes at the wavelengths used39. The absence of Raman signals from the MoS2 flake under the Ti/Au electrodes in Fig. 3b implies, therefore, that the observed photocurrent inside the electrodes could not arise from PVE at the MoS2-metal interface under the electrodes. Another photocurrent mechanism than PVE should be at work.


Photothermoelectric and photovoltaic effects both present in MoS2.

Zhang Y, Li H, Wang L, Wang H, Xie X, Zhang SL, Liu R, Qiu ZJ - Sci Rep (2015)

Photocurrent and Raman images of MoS2 transistor.(a) Short-circuit scanning photocurrent images taken at Vg = 15 V with a laser power of 100 μW. (b) Raman mapping image with the normalized E12g peak intensity of the MoS2 multilayer. In panels (a) and (b), the dashed lines indicate the edges of the source and drain electrodes while the white lines mark the boundaries of MoS2. (c) Energy-band diagram of a multilayer- MoS2 transistor under light illumination in the accumulation (Vg > 0 V) and depletion (Vg < 0 V) regimes. Electron-hole pairs are generated in the space-charge region by light absorption and contribute to PVE and PTE photocurrents, which are indicated by blue and red arrows, respectively. (d) Gate-dependent Seebeck coefficient (S, black line) and the open-circuit photovoltage (VPH, red line) with a 100-μW laser illuminated at the contact edge. The vertical blue dotted lines separate the regimes of accumulation, depletion and inversion while the horizontal black dashed line indicates the border between the electron and hole conductions.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4300500&req=5

f3: Photocurrent and Raman images of MoS2 transistor.(a) Short-circuit scanning photocurrent images taken at Vg = 15 V with a laser power of 100 μW. (b) Raman mapping image with the normalized E12g peak intensity of the MoS2 multilayer. In panels (a) and (b), the dashed lines indicate the edges of the source and drain electrodes while the white lines mark the boundaries of MoS2. (c) Energy-band diagram of a multilayer- MoS2 transistor under light illumination in the accumulation (Vg > 0 V) and depletion (Vg < 0 V) regimes. Electron-hole pairs are generated in the space-charge region by light absorption and contribute to PVE and PTE photocurrents, which are indicated by blue and red arrows, respectively. (d) Gate-dependent Seebeck coefficient (S, black line) and the open-circuit photovoltage (VPH, red line) with a 100-μW laser illuminated at the contact edge. The vertical blue dotted lines separate the regimes of accumulation, depletion and inversion while the horizontal black dashed line indicates the border between the electron and hole conductions.
Mentions: To identify the mechanism of photocurrent generation, SPCM was conducted at zero source-drain bias (Vd = 0 V) with a well-focused laser beam scanning over the device area. The short-circuit photocurrent and Raman spectra were recorded simultaneously as a function of laser position. To avoid possible photo-damage to the MoS2 during experiment35, the step size was set to be 1 μm. Figure 3a shows the photocurrent image measured at Vg = 15 V, where two photocurrent extrema of opposite polarities are observed at the electrode edges. As expected from conventional PVE, the opposite built-in electric fields from space charges at the source/drain contacts create an asymmetrical photocurrent characteristic. Such PVE-induced photocurrents localized near electrodes have been widely reported in devices based on silicon nanowires36 and carbon nanotubes3738. However, it is remarkable to observe a significant photocurrent when the laser spot is centered inside the electrodes. The distance from the electrode edge is ~10 times larger than the laser spot size in order to make sure that the MoS2 channel is not illuminated. Besides, the light penetration depth is smaller than the thickness for the metal electrodes at the wavelengths used39. The absence of Raman signals from the MoS2 flake under the Ti/Au electrodes in Fig. 3b implies, therefore, that the observed photocurrent inside the electrodes could not arise from PVE at the MoS2-metal interface under the electrodes. Another photocurrent mechanism than PVE should be at work.

Bottom Line: The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE).In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime.Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.

View Article: PubMed Central - PubMed

Affiliation: 1] State Key Laboratory of ASIC &System, School of Information Science and Technology, Fudan University, Shanghai 200433, China [2] State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem &Information Technology, Chinese Academy of Sciences, Changning Road 865, Shanghai 200050, China.

ABSTRACT
As a finite-energy-bandgap alternative to graphene, semiconducting molybdenum disulfide (MoS2) has recently attracted extensive interest for energy and sensor applications. In particular for broad-spectral photodetectors, multilayer MoS2 is more appealing than its monolayer counterpart. However, little is understood regarding the physics underlying the photoresponse of multilayer MoS2. Here, we employ scanning photocurrent microscopy to identify the nature of photocurrent generated in multilayer MoS2 transistors. The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE). In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime. Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.

No MeSH data available.


Related in: MedlinePlus