The photovoltaic effect in traditional p–n junctions—where a p-type material (with an excess of holes) abuts an n-type material (with an excess of electrons)—involves the light-induced creation of electron–hole pairs and their subsequent separation, generating a current. This photovoltaic effect is particularly important for environmentally benign energy harvesting, and its efficiency has been increased dramatically, almost reaching the theoretical limit1. Further progress is anticipated by making use of the bulk photovoltaic effect (BPVE)2, which does not require a junction and occurs only in crystals with broken inversion symmetry3. However, the practical implementation of the BPVE is hampered by its low efficiency in existing materials4,5,6,7,8,9,10. Semiconductors with reduced dimensionality2 or a smaller bandgap4,5 have been suggested to be more efficient. Transition-metal dichalcogenides (TMDs) are exemplary small-bandgap, two-dimensional semiconductors11,12 in which various effects have been observed by breaking the inversion symmetry inherent in their bulk crystals13,14,15, but the BPVE has not been investigated. Here we report the discovery of the BPVE in devices based on tungsten disulfide, a member of the TMD family. We find that systematically reducing the crystal symmetry beyond mere broken inversion symmetry—moving from a two-dimensional monolayer to a nanotube with polar properties—greatly enhances the BPVE. The photocurrent density thus generated is orders of magnitude larger than that of other BPVE materials. Our findings highlight not only the potential of TMD-based nanomaterials, but also more generally the importance of crystal symmetry reduction in enhancing the efficiency of converting solar to electric power.
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The data that support the findings of this study are available from the corresponding author upon reasonable request.
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We thank M. Kuehne, H. Isobe, M. Nakamura, N. Ogawa and D. Zhao for discussions; S. Göres Y. Stuhlhofer, J. Geurs and Y. Kim for technical assistance; and A. Oiwa, B. Zhang, K. von Klitzing, Y. Tokura, M. Kawasaki and N. Nagaosa for suggestions. We acknowledge financial support from the Japan Society for the Promotion of Science (to Y.J.Z., M.O. and R.S. through the research fellowship program for young scientists; M.O. through the Advanced Leading Graduate Course for Photon Science; T.I. through the Challenging Research (Exploratory) (no. JP17K18748), the ‘Topological Materials Science’ (no. JP18H04216) KAKENHI on Innovative Areas, and Scientific Research (B) (no. JP19H01819); and Y.I. through a grant-in-aid for specially promoted research (no. 25000003) and Scientific Research (A) (no. JP19H00653)). We also acknowledge financial support from the Israel Science Foundation (ISF; to A.Z. and R.T.; no. 330/16 and 339/18); the Pazy Foundation of Israel (to A.Z.); the H. Perlman and the Irving and Azelle Waltcher Foundations in honour of M. Levy (to R.T.); the H. Perlman family foundation (to R.T.; no. 720821); and the Graphene Flagship (to J.H.S.).
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
a, The black trace shows the results of an X-ray diffraction analysis of a WS2 nanotube. The red bars indicate the simulated peak positions for a 2H-type bulk WS2 material. The small shift in the (002), (004) and (006) peaks for the WS2 nanotube is also consistent with previous work51 indicating that the distance between layers in the nanotubes is slightly larger than that in bulk WS2. b, Electron-diffraction pattern for a WS2 nanotube. White and blue hexagons have been included to indicate the differing chirality of each layer making up the nanotube wall19. The white scale bar represents 5 nm−1. The red arrow marks the orientation of the nanotube, as determined from the TEM image in the inset (black scale bar, 50 nm). c, Colour rendition of the height map recorded by AFM on devices 1 and 2. Scale bars represent 4 µm. d, Height profiles of both tubes along the dashed lines in panel c. The horizontal axis (x) represents the position along these lines. The diameter of the first nanotube (device 1) is 180 nm; for device 2, the diameter is 90 nm. a and b are adapted from ref. 19.
a, Dependence of Isc on the position of the laser spot. The scale bar in the optical micrograph of the device represents 4 µm. The horizontal axis (x) shows the distance between the laser spot and one of the electrodes (see optical micrograph, inset). b, DC I–V characteristics in the dark at voltages up to ±1 V.
Extended Data Fig. 3 Effect of oblique incidence of the laser light on the photocurrent response of a WS2 nanotube device.
a, Illustration of the oblique incidence condition in WS2 nanotubes. b, Dependence of the photocurrent on the polarization characteristics of the incident laser beam.
Extended Data Fig. 4 AFM and SKPM of WS2 nanotubes, and influence of domain number and size on the net Dember photovoltaic effect.
a, Colour rendition of a height map recorded by AFM in the dark. The scale bar represents 2 µm. b, Greyscale map of the surface potential recorded with a scanning Kelvin probe microscope in the dark. The scanned area is identical to that in a. The sample contacts are both connected to ground during the measurement. c, Line scans of the surface potential along three different WS2 nanotubes. d, Expected spatial dependence of the photovoltaic signal caused by the Dember effect for three different domain configurations. Domains with different electrical characteristics are coloured differently and marked with δ+ (dark green) or δ− (light green).
The short-circuit current density, jsc, is plotted against laser power for bulk ferroelectric materials and WS2 nanotubes. Filled symbols are used for data points obtained in a lateral device configuration, whereas open symbols refer to data points obtained on other, vertical device geometries. References are as follows: KBNNO4; TTF-CA5; Pb(ZrTi)O3, lateral6, vertical47, contact with copper oxide48; BaTiO3, lateral6, vertical49; BiFeO3, lateral7, tip enhancement50; MaPbI3, lateral and vertical8; organometallic perovskite halide9; BFCO10.
The laser wavelength was 532 nm. a, I–V characteristics recorded with a ×10 objective. b, Dependence of Isc on the linear polarization angle of the incident laser beam. c, Laser power dependence of Isc. The dashed lines correspond to a linear and a square-root power dependence and serve as guides to the eye. d, Temperature dependence of Voc. The laser power density was set to 8.29 × 102 W cm−2 for a, b and d. The data in a–c were recorded at 300 K.
The incident laser beam was periodically modulated at a frequency of 133 Hz with the help of a chopper. The excitation wavelength and laser power were 632.8 nm and 6.32 × 103 W cm−2, respectively. a, Dependence of Isc on the position of the laser spot. The scale bar in the optical micrograph of the device represents 4 µm. Both magnitude and phase are plotted. The horizontal axis (x) is the distance between the laser spot and one of the electrodes (see optical micrograph). b, Dependence of the photocurrent on the linear polarization angle of the incident laser beam. The thick black arrows at the centre of the polar diagrams mark the orientation of the nanotube. c, Magnitude and phase of the AC photocurrent as a function of the applied voltage.