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Optical Imaging of Charges with Atomically Thin Molybdenum Disulfide Hao Zhu, Fenni Zhang, Hui Wang, Zhixing Lu, Hong-yuan Chen, Jinghong Li, and Nongjian Tao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09010 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019
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ACS Nano
Optical Imaging of Charges with Atomically Thin Molybdenum Disulfide Hao Zhu,† Fenni Zhang,‡ Hui Wang,† Zhixing Lu,§ Hong-yuan Chen,*,† Jinghong Li,*,§ and Nongjian Tao*,†,‡ †
State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210023, China ‡
Center for Bioelectronics and Biosensors, Biodesign Institute, Arizona State University, Tempe,
Arizona 85287, United States. §
Department of Chemistry, Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical
Biology, Tsinghua University, Beijing 100084, China.
ABSTRACT: Mapping local surface charge distribution is critical to the understanding of various surface processes and also allows the detection of molecules binding to the surface. We show here that the optical absorption of monolayer MoS2 is highly sensitive to charge and demonstrate optical imaging of local surface charge distribution with this atomically thin material. We validate the imaging principle and perform charge sensitivity calibration with an electrochemical gate. We further show that binding of charged molecules to the atomically thin material leads to a large change in the image contrast, allowing determination of the charge of the adsorbed molecules. This
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capability opens possibilities for characterizing impurities and defects in two dimensional materials and for label-free optical detection and charge analysis of molecules. KEYWORDS: monolayer molybdenum disulfide, electrochemical gate, charged impurities, local charge imaging, protein binding.
Charge is a fundamental property that plays an essential role in various physical, chemical and biological processes.1–4 By detecting the charges of molecules, methods have been developed to measure molecular binding to surfaces for biosensing applications.5,6 One example is field effect transistors (FETs) based on traditional semiconductors or nanomaterials for detecting molecules and sequencing DNA.7–10 Another example is the detection of charged molecules with an optical fiber, which has led to label-free detection of small charged drug molecules in a microplate compatible format.11,12 A distinct advantage of this charge-based detection approach over the traditional mass-based detection technologies is that its sensitivity does not scale with molecular weight, making it suitable to detect both large and small molecules. However, these methods do not offer imaging capability, which are thus unable to resolve local charges on a surface. To obtain the spatial distribution of surface charges, the scanning probe microscopes, such as the atomic force microscope (AFM),13–15 scanning gate microscope (SGM)16,17 and scanning ion conductance microscope (SICM)18,19 have been developed. These methods provide high spatial resolution but scanning a probe across the surface mechanically is slow and complicated, which are undesirable for biosensing applications. We have developed plasmonic microscopy to map surface charges,20,21 which allows detection of molecules and imaging of local impurities in graphene.22,23 The plasmonic imaging method relies on the dependence of surface plasmon resonance frequency of a metal film on charge. Because the metal film is thick (hundreds of atomic
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ACS Nano
layers) and its electron density is high and un-tunable, the charge sensitivity of the plasmonic imaging technique remains limited. Here we report a method to image local charges with monolayer molybdenum disulfide (MoS2) optically. This two-dimensional (2D) transition metal dichalcogenide is atomically thin with carrier density tunable by applying a gate voltage.24–27 Studies have shown that monolayer MoS2 has a band gap of 1.9 eV, leading to optical absorption in the visible range.28,29 More importantly, the optical absorption is sensitive to charge carrier density. We show that this unique property of monolayer MoS2 can be used to image charges with charge sensitivity more than one order of magnitude greater than the plasmonic imaging. To investigate the charge sensitivity, we measured optical transmission images of monolayer MoS2 in an electrolyte by applying an electrochemical gate voltage to the 2D material. We then imaged the local optical response to an alternating voltage superimposed on the gate voltage and determined the dependence of the image contrast vs the gate voltage, amplitude and frequency of the alternating gate voltage. These images are chargesensitive, allowing mapping of local charges in the monolayer MoS2. We further demonstrate that binding of bovine serum albumin (BSA) on monolayer MoS2 leads to a change in the image contrast, from which the charge of the protein is determined. We finally compare the charge sensitivity of monolayer MoS2 with thin gold films, both with direct optical transmission and plasmonic imaging approaches.
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Figure 1. Principle and experimental setup. (a) Schematic illustration of the experimental setup, where WE, RE, and CE are the working, reference, and counter electrodes, respectively. (b) Absorbance spectrum of the monolayer MoS2. (c) Measured transmission change ∆𝑻/𝑻 of both the monolayer MoS2 and bare ITO regions as a function of gate voltage (𝑽𝐠 ) in 0.1 M NaF aqueous solution. (d) Raman spectrum of the monolayer MoS2 sample. (e) Photoluminescence (PL) spectrum of the sample. (f) Optical transmission image of the monolayer MoS2 on the ITO slide. (g–j) Differential images at gate voltages of 0.4 V (g), −0.1 V (h), −0.2 V (i) and −0.4 V (j). Each of the differential images was obtained by subtracting the image at 1.0 V. Image scale bars in (f–j): 5 μm.
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ACS Nano
RESULTS AND DISCUSSION Principle and experimental setup. Monolayer MoS2 absorbs visible light with peaks near ~615 nm and 670 nm (Figure 1b),29 which arise from excitons and trions.28,30 The peak heights are sensitive to the carrier density in MoS2.25 Consequently, the presence of charges on the MoS2 surface induces a change in the local carrier density, which is translated into a change in the optical absorption. By measuring the local optical absorption, we can thus image the local charge distribution on the MoS2 surface. Binding of charged molecules on the MoS2 monolayer changes the surface charge, which is imaged by measuring local optical transmission. For a small change in the surface charge density (∆𝑞), the corresponding optical transmission change (∆𝑇/𝑇) may be expressed as ∆𝑇 𝑇
= 𝛼∆𝑞,
(1)
where α is a calibration factor. Once α is calibrated, the local charge density image is determined from the optical transmission image. We validated the charge imaging principle, performed the calibration and demonstrated detection of molecular binding by placing the monolayer MoS2 on an indium tin oxide (ITO) slide mounted on an inverted optical microscope (Figure 1a). The monolayer MoS 2 was immersed in 0.1 M NaF aqueous solution and its carrier density was controlled by applying a gate voltage with respect to a Ag/AgCl reference electrode inserted the electrolyte. Gating efficiency in field effect transistors is determined by the effective distance between the gate electrode and the material. In the electrochemical-gating configuration, the effective distance is the thickness of the electrical double layer, approximately the size of a hydrated ion (
