COMMUNICATION 1704412 (1 of 6) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de Number-Resolved Single-Photon Detection with Ultralow Noise van der Waals Hybrid Kallol Roy,* Tanweer Ahmed, Harshit Dubey, T. Phanindra Sai, Ranjit Kashid, Shruti Maliakal, Kimberly Hsieh, Saquib Shamim, and Arindam Ghosh* K. Roy, T. Ahmed, H. Dubey, Dr. T. P. Sai, Dr. R. Kashid, [+] K. Hsieh, Dr. S. Shamim, [++] Prof. A. Ghosh Department of Physics Indian Institute of Science Bangalore 560012, India E-mail: kallol@iisc.ac.in; arindam@iisc.ac.in S. Maliakal Department of Physics Indian Institute of Science Education and Research Mohali 140306, India Prof. A. Ghosh Centre for Nano Science and Engineering Indian Institute of Science Bangalore 560012, India DOI: 10.1002/adma.201704412 state incident at internal quantum effi- ciency η. A real photodetector, however, suffers from device-related noise, predom- inantly from receiver electronics and/or randomness in the feedback process when the device, such as a photomultiplier tube (PMT), or an avalanche photodiode (APD), is operated at a large optoelectronic gain (G). The combined contributions to noise lead to [12] SNR / 2 q 2 2 m mF G σ = 〈 〉 〈 〉 + 〈 〉 (1) where F = 〈G 2 〉/〈G〉 2 and σ q are the excess gain noise and dimensionless receiver circuit noise, respectively. Equation (1) indicates that large G helps in detecting small number of photons (see schematic in Figure 1a), but rapid increase in F for G > 100–1000 imposes a practical limitation in conventional APDs made from bulk semiconductor heterojunctions. [12] In ultrathin-layered semiconductors from TMDCs, for example, MoS 2 , WS 2 , WSe 2 etc., van Hove singularities in the electronic density of states enable remarkably strong light absorption even when the thickness of the semiconductors is reduced to single molecular layers. [4] This makes single- or few- layer MoS 2 an excellent photodetector itself, [13] but low carrier mobility, sensitivity to environmental charge fluctuations, and localization of carriers at the band tail are detrimental in high- resolution photodetection. [14,15] The planar van der Waals hybrids of graphene and TMDCs are emerging class of photodetec- tors, [4,5,16] where the light to electricity conversion is obtained by interlayer charge transfer and an effective photogating of the graphene channel. These detectors leverage the large car- rier mobility of graphene, and high interfacial transparency due to small graphene/MoS 2 Schottky barrier (φ B ≈ 0.2–0.4 eV), [17] and both conceptually and architecturally distinct from the bulk semiconductor-based photon counting devices, such as the APDs. [18–21] For example, in contrast to impact ionization in APDs, the carrier multiplication in the hybrid occurs in the graphene channel under electrostatic influence of the photo- generated hole located in MoS 2 (see Figure 1b,c). The gra- phene layer thus functions both as the multiplier and the receiver circuit. Separating the photogeneration region (MoS 2 ) enhances lifetime of the hole to τ lifetime ≈ 1 s, [5] so that the opto- electronic gain G = τ lifetime /τ tr ≈ 10 10 is intrinsically very large (τ tr ≈ 0.1 ns is the transport time of carriers in the graphene channel). Moreover, as illustrated in Figure 1b, the dominant gain-noise mechanism in APDs, such as stochasticity in carrier Van der Waals hybrids of graphene and transition metal dichalcogenides exhibit an extremely large response to optical excitation, yet counting of photons with single-photon resolution is not achieved. Here, a dual-gated bilayer graphene (BLG) and molybdenum disulphide (MoS 2 ) hybrid are demonstrated, where opening a band gap in the BLG allows extremely low channel (receiver) noise and large optical gain (≈10 10 ) simultaneously. The resulting device is capable of unambiguous determination of the Poissonian emission statistics of an optical source with single-photon resolution at an operating temperature of 80 K, dark count rate 0.07 Hz, and linear dynamic range of ≈40 dB. Single-shot number-resolved single-photon detection with van der Waals heterostructures may impact multiple technologies, including the linear optical quantum computation. Van der Waals Materials Determining the number of photons in an optical pulse with single-photon resolution is the ultimate goal in photodetection. [1,2] Planar hybrids of graphene and transition metal dichalcogenides (TMDC) [3–8] are unique optoelectronic elements where coherent transfer of photogenerated electrons across the van der Waals interface [7,9,10,11] results in extremely large gain at low feedback noise. [5,6,8] In an ideal photon counting device, the signal-to-noise ratio (SNR) is limited by emission statistics of the optical source. The resulting electrical output fluctuates with SNR / 2 2 m m σ =〈 〉 , and simplifies to SNR = 〈m〉, for purely Poissonian emission probability P n (μ) = (μ n /n!)e −μ , where μ = 〈n〉, and m(=ηn) and σ m are the number and vari- ance, respectively, of the photocarriers generated for n-photon [+] Present address: National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg, MD 20899, USA [++] Present address: Physikalisches Institut (EP3), Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany Adv. Mater. 2017, 1704412