The development of infrared photodetectors is mainly limited by the choice of available materials and the intricate crystal growth process. Moreover, thermally activated carriers in traditional III–V and II–VI semiconductors enforce low operating temperatures in the infrared photodetectors. Here we demonstrate infrared photodetection enabled by interlayer excitons (ILEs) generated between tungsten and hafnium disulfide, WS2/HfS2. The photodetector operates at room temperature and shows an even higher performance at higher temperatures owing to the large exciton binding energy and phonon-assisted optical transition. The unique band alignment in the WS2/HfS2 heterostructure allows interlayer bandgap tuning from the mid- to long-wave infrared spectrum. We postulate that the sizeable charge delocalization and ILE accumulation at the interface result in a greatly enhanced oscillator strength of the ILEs and a high responsivity of the photodetector. The sensitivity of ILEs to the thickness of two-dimensional materials and the external field provides an excellent platform to realize robust tunable room temperature infrared photodetectors.
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The work is supported by the Agency for Science, Technology and Research (A*STAR) under the 2D Materials Pharos Program (grant no. 152 700014 and grant no. 152 700017). Q.J.W. acknowledges funding from the National Research Foundation Competitive Research Program (NRF-CRP18-2017-02 and NRF–CRP19–2017–01). K.S.T. acknowledges support from the Center for Nanostructured Graphene (CNG) under the Danish National Research Foundation (project DNRF103) and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant no. 773122, LIMA). G.L. acknowledges the supported under the grant MOE2017-T2-1-114. H.L. acknowledges the support by the Ministry of Science and Technology (MOST) in Taiwan under grant no. MOST 109-2112-M-001-014-MY3. J.T. and S. Lukman thank C. W. Lee, A. Ngo and M. Zhao for their valuable inputs and K Hippalgaonkar for sharing tools in the device fabrication.
The authors declare no competing interests.
Peer review information Nature Nanotechnology thanks Yanqing Wu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Absorption spectra of ILE at heterointerface between monolayer of WS2 and N-layer(s) of HfS2 measured at room temperature. Source data
The variation of interlayer exciton (ILE) energy determined by fitting the absorption peak to Gaussian function, in the absence and presence of gate voltage Vg. The plotted value is the center of the Gaussian and the standard deviation corresponds to the error from the fitting. The shaded bars represent the spot-to-spot and sample-to-sample variation. Source data
a, Photoluminescence (PL) spectra of WS2 monolayer with N-layers of HfS2. Peak splitting of the photoluminescence spectra is present at N = 2,3,4, and 5. Only a single Gaussian peak is present for N = 1 and bulk HfS2 (N > 5). The observed peak splitting is the result of coupling-induced CBM splitting in HfS2. The plot is shifted in y-axis for clarity b, Comparison between experimental and calculated energy level splitting in CBM as function of number of layers of HfS2. Source data
Integrated photoluminescence (PL) intensity of monolayer WS2 exciton and interlayer exciton generated at the heterointerface between WS2 and HfS2 (3 layers and bulk). In WS2-3L HfS2, the PL peaks are doublet, each is fitted independently to Gaussian and the area under the curve is plotted independently as ILElow and ILEhigh. Source data
Extended Data Fig. 5 Band profiles at the interface of WS2 and HfS2 in the presence of external bias.
Band profiles of 3L-, 5L- and 7L-HfS2/1L-WS2 heterojunction in the presence of external bias a, 0.3 V, and b, 0.5 V, respectively. The energy reference is set to be the vacuum level (Evac = 0 eV). The highlighted grey-area is the space between the WS2 and HfS2. The quasi Fermi level EFL and EFR of WS2 and HfS2 respectively are plotted in black dash line. The black dots denote the location of atoms in out-of-plane direction. WS2 is slightly n-doped, with the Fermi energy level 0.15 eV above the intrinsic Fermi level (Egap = 1.82 eV).
Dark current in photodetectors made of WS2 and 3 layers HfS2 a, and bulk HfS2 b, at two different gating voltage (−40V and + 40 V) and Vds = −1.5 V. Source data
Normalised rise and fall of photocurrent in WS2-HfS2 device (Vg = 0 V, Vds = −1.5 V) under a modulated illumination source with λ = 4.7 µm and Vg = 0 V. The photoresponse time is determined by exponential fitting to the time required for signal to rise from 10-90% and fall from 90–10% under illumination source (Pdevice ~ 0.5 ± 0.3 nW). The rise and fall times for WS2-3L HfS2 is 1.7 ms and 2.2 ms, respectively. The rise and fall times for WS2-bulk HfS2 is 3.4 ms and 3.6 ms, respectively. Source data
Normalised rise and fall of photocurrent in one of the WS2-HfS2 device (Vg = 0 V, Vds = −1.5 V) under a modulated illumination source of different wavelength, Vg = 0 V, and Pdevice ~ 0.5 ± 0.3 nW. The response time does not change much with illumination wavelength. Source data
Normalised absorption spectra in IR range for the WS2-HfS2 HSs as function of externally applied gate voltage (Vg). The relative bandgap of WS2 VBM and HfS2 CBM decreases with more negative bias and vice versa. The shift is more prominent in WS2- bulk HfS2 owing to higher exciton density at the interface. Source data
Potential improvement on the photodetector based on ILE via configurations such as a, ABA or b, ABC stacking. The former would behave like a ‘quantum well’ and enhance the efficiency of charge hopping and thus higher efficiency, whereas the latter is good for multiple-wavelength photo-detectability.
Compiled raw numerical data used in Fig. 1 in the main text (Origin file under ‘Main’ sub folder).
Compiled raw numerical data used in Fig. 2 in the main text (Origin file under ‘Main’ sub folder).
Compiled raw numerical data used in Fig. 3 in the main text (Origin file under ‘Main’ sub folder).
Compiled raw numerical data used in Fig. 4 in the main text (Origin file under ‘Main’ sub folder)..
Compiled raw numerical data used in Fig. 5 in the main text (Origin file under ‘Main’ sub folder)
Compiled raw numerical data used in Fig. 1 in Extended Data (Origin file under ‘Extended’ sub folder).
Compiled raw numerical data used in Fig. 2 in Extended Data (Origin file under ‘Extended’ sub folder).
Compiled raw numerical data used in Fig. 3 in Extended Data (Origin file under ‘Extended’ sub folder).
Compiled raw numerical data used in Fig. 4 in Extended Data (Origin file under ‘Extended’ sub folder).
Compiled raw numerical data used in Fig. 6 in Extended Data (Origin file under ‘Extended’ sub folder).
Compiled raw numerical data used in Fig. 7 in Extended Data (Origin file under ‘Extended’ sub folder).
Compiled raw numerical data used in Fig. 8 in Extended Data (Origin file under ‘Extended’ sub folder).
Compiled raw numerical data used in Fig. 9 in Extended Data (Origin file under ‘Extended’ sub folder).
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Lukman, S., Ding, L., Xu, L. et al. High oscillator strength interlayer excitons in two-dimensional heterostructures for mid-infrared photodetection. Nat. Nanotechnol. (2020). https://doi.org/10.1038/s41565-020-0717-2