Abstract:One-dimensional nanoscale devices, such as semiconductor nanowires (NWs) and singlewalled carbon nanotubes (SWNTs), have been intensively investigated because of their potential application of future high-speed electronic, optoelectronic, and sensing devices 1-3 .To overcome current limitations on the speed of contemporary devices, investigation of charge carrier dynamics with an ultrashort time scale is one of the primary steps necessary for developing high-speed devices. In the present study, we visualize ul… Show more
“…At zero gate bias, we generally observed a negative photocurrent near the drain electrode and a positive current near the source electrode. This implies that it is the hole carriers that are collected by the metal electrodes, as opposed to the previous results on various semiconducting devices with p-type bending near the electrode 27–30 . This is also in contrast to the bare STO devices, which exhibit p-type SPCM signals, as shown in Fig.…”
Section: Resultscontrasting
confidence: 81%
“…2b. This implies that it is the photogenerated electron carriers that are collected by the metal electrodes, as in the case of previous results on various semiconducting devices with p-type bending near the electrode 27–30 .Figure 2( a ) SPCM of a bare STO device at V
SD = V
G = 0 ( b ) Photocurrent as a function of V
G near the drain (red) and source (blue) electrodes. (inset) Schematic band diagrams of STO device for p-type (left), and n-type (right) operation.…”
Section: Resultsmentioning
confidence: 52%
“…However, our understanding of the gate-dependence of the SPCM signals with respect to the electronic band alignment is limited, because we could not observed the gate-response in the DC conductance measurement for the bare-STO devices that act as insulators in the absence of UV illumination. Conversely, the electronic band structure of the Al 2 O 3 /STO heterointerface can be studied in detail by using the SPCM signals at the metal contacts in conjunction with gate-dependent transport measurements 27–30 .…”
Section: Resultsmentioning
confidence: 99%
“…Over the past decade, scanning photocurrent microscopy (SPCM) has been successfully used to study the electrical and photoelectrical properties of various nanoscale devices 26–30 . Information obtained from SPCM includes the band structure of the metallic contacts, junctions, and defects.…”
The band alignment at an Al2O3/SrTiO3 heterointerface forming a two-dimensional electron gas (2DEG) was investigated using scanning photocurrent microscopy (SPCM) in an electrolyte-gated environment. We used a focused UV laser source for above-the-bandgap illumination on the SrTiO3 layer, creating electron-hole pairs that contributed to the photocurrent through migration towards the metal electrodes. The polarity of the SPCM signals of a bare SrTiO3 device shows typical p-type behavior at zero gate bias, in which the photogenerated electrons are collected by the electrodes. In contrast, the SPCM polarity of 2DEG device indicates that the hole carriers were collected by the metal electrodes. Careful transport measurements revealed that the gate-dependent conductance of the 2DEG devices exhibits n-type switching behavior. More importantly, the SPCM signals in 2DEG devices demonstrated very unique gate-responses that cannot be found in conventional semiconducting devices, based on which we were able to perform detailed investigation into the electronic band alignment of the 2DEG devices and obtain the valence band offset at the heterointerface.
“…At zero gate bias, we generally observed a negative photocurrent near the drain electrode and a positive current near the source electrode. This implies that it is the hole carriers that are collected by the metal electrodes, as opposed to the previous results on various semiconducting devices with p-type bending near the electrode 27–30 . This is also in contrast to the bare STO devices, which exhibit p-type SPCM signals, as shown in Fig.…”
Section: Resultscontrasting
confidence: 81%
“…2b. This implies that it is the photogenerated electron carriers that are collected by the metal electrodes, as in the case of previous results on various semiconducting devices with p-type bending near the electrode 27–30 .Figure 2( a ) SPCM of a bare STO device at V
SD = V
G = 0 ( b ) Photocurrent as a function of V
G near the drain (red) and source (blue) electrodes. (inset) Schematic band diagrams of STO device for p-type (left), and n-type (right) operation.…”
Section: Resultsmentioning
confidence: 52%
“…However, our understanding of the gate-dependence of the SPCM signals with respect to the electronic band alignment is limited, because we could not observed the gate-response in the DC conductance measurement for the bare-STO devices that act as insulators in the absence of UV illumination. Conversely, the electronic band structure of the Al 2 O 3 /STO heterointerface can be studied in detail by using the SPCM signals at the metal contacts in conjunction with gate-dependent transport measurements 27–30 .…”
Section: Resultsmentioning
confidence: 99%
“…Over the past decade, scanning photocurrent microscopy (SPCM) has been successfully used to study the electrical and photoelectrical properties of various nanoscale devices 26–30 . Information obtained from SPCM includes the band structure of the metallic contacts, junctions, and defects.…”
The band alignment at an Al2O3/SrTiO3 heterointerface forming a two-dimensional electron gas (2DEG) was investigated using scanning photocurrent microscopy (SPCM) in an electrolyte-gated environment. We used a focused UV laser source for above-the-bandgap illumination on the SrTiO3 layer, creating electron-hole pairs that contributed to the photocurrent through migration towards the metal electrodes. The polarity of the SPCM signals of a bare SrTiO3 device shows typical p-type behavior at zero gate bias, in which the photogenerated electrons are collected by the electrodes. In contrast, the SPCM polarity of 2DEG device indicates that the hole carriers were collected by the metal electrodes. Careful transport measurements revealed that the gate-dependent conductance of the 2DEG devices exhibits n-type switching behavior. More importantly, the SPCM signals in 2DEG devices demonstrated very unique gate-responses that cannot be found in conventional semiconducting devices, based on which we were able to perform detailed investigation into the electronic band alignment of the 2DEG devices and obtain the valence band offset at the heterointerface.
“…All dynamics that occur after this (propagation of the potential to the contacts, signals travelling through the cables, and so on) do not affect the results of our specific experiment. To measure transit times, one can use two-pulse excitation with two different focus spots 35 .…”
*Graphene is a promising material for ultrafast and broadband photodetection. Earlier studies have addressed the general operation of graphene-based photothermoelectric devices and the switching speed, which is limited by the charge carrier cooling time, on the order of picoseconds. However, the generation of the photovoltage could occur at a much faster timescale, as it is associated with the carrier heating time. Here, we measure the photovoltage generation time and find it to be faster than 50 fs. As a proof-of-principle application of this ultrafast photodetector, we use graphene to directly measure, electrically, the pulse duration of a sub-50 fs laser pulse. The observation that carrier heating is ultrafast suggests that energy from absorbed photons can be efficiently transferred to carrier heat. To study this, we examine the spectral response and find a constant spectral responsivity of between 500 and 1,500 nm. This is consistent with efficient electron heating. These results are promising for ultrafast femtosecond and broadband photodetector applications.
Photovoltage generation through the photothermoelectric (PTE) effect occurs when light is focused at the interface of monolayer and bilayer graphene, or at the interface between regions of graphene with different Fermi energies E F (refs 1-6). In such graphene PTE devices-which operate over a large spectral range 7,8 that extends even into the far-infrared 9 -local heating of electrons by absorbed light, in combination with a difference in Seebeck coefficients between the two regions, gives rise to a PTE voltage V PTE = (S 2 − S 1 )(T el − T 0 ). Here, S 1 and S 2 are the Seebeck coefficients of regions 1 and 2, respectively, T el is the hot electron temperature after photoexcitation and electron heating, and T 0 is the temperature of the electrode heat sinks. The performance of PTE graphene devices is intimately connected to the dynamics of the photoexcited electrons and holes, which have mainly been studied in graphene samples through ultrafast optical pumpprobe measurements [10][11][12][13][14][15][16][17] . As shown in Fig. 1a, the dynamics start with (i) photoexcitation and electron-hole pair generation, followed by (ii) electron heating through carrier-carrier scattering, in competition with lattice heating, both of which take place on a sub-100 fs timescale, and finally (iii) electron cooling by thermal equilibration with the lattice, which takes place on a picosecond timescale. The effect of the picosecond cooling step (iii) on the switching speed of graphene devices has been studied using timeresolved photovoltage scanning experiments with ∼200 fs time resolution [18][19][20] . These studies showed that the picosecond electron cooling time limits the intrinsic photo-switching rate of these devices to a few hundred gigahertz, because faster switching would reduce the switching contrast, as the system does not have time to return to the ground state. Indeed, gigahertz switching speeds have been demonstrated in graphene-based devices [21][22][23]...
We directly observed charge separation and a space‐charge region in an organic single‐crystal p–n heterojunction nanowire, by means of scanning photocurrent microscopy. The axial p–n heterojunction nanowire had a well‐defined planar junction, consisted of P3HT (p‐type) and C60 (n‐type) single crystals and was fabricated by means of the recently developed inkjet‐assisted nanotransfer printing technique. The depletion region formed at the p–n junction was directly observed by exploring the spatial distribution of photogenerated carriers along the heterojunction nanowire under various applied bias voltages. Our study provides a facile approach toward the precise characterization of charge transport in organic heterojunction systems as well as the design of efficient nanoscale organic optoelectronic devices.
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