Microsteganography on WS2 Monolayers Tailored by Direct Laser Painting Ashwin Venkatakrishnan,†,# Hou Chua,†,# Pinxi Tan,†,# Zhenliang Hu,‡ Hongwei Liu,§ Yanpeng Liu,∥ Alexandra Carvalho,*,∥ Junpeng Lu,*,‡,∥ and Chorng Haur Sow*,‡,∥ †
NUS High School of Mathematics and Science, 20 Clementi Avenue 1, Singapore 129957 Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 § Institute of Materials Research and Engineering, Agency for Science, Technology, and Research (A*STAR), 2 Fusionopolis Way, lnnovis, #08-03, Singapore 138634 ∥ Center for Advanced 2D Materials and Graphene Research Center, National University of Singapore, 6 Science Drive 3, Singapore 117546 ‡
S Supporting Information *
ABSTRACT: We present scanning focused laser beam as a multipurpose tool to engineer the physical and chemical properties of WS2 microflakes. For monolayers, the laser modification integrates oxygen into the WS2 microflake, resulting in ∼9 times enhancement in the intensity of the fluorescence emission. This modification does not cause any morphology change, allowing “micro-encryption” of information that is only observable as fluorescence under excitation. The same focused laser also facilitates on demand thinning down of WS2 multilayers into monolayers, turning them into fluorescence active components. With a scanning focused laser beam, micropatterns are readily created on WS2 multilayers through selective thinning of specific regions on the flake. KEYWORDS: laser modification, micropatterning, fluorescence, WS2 TMDs
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(CVD) methods would affect the optical quality of the monolayers. Recently, various approaches have been developed to improve the luminescence efficiency of TMDs monolayers. For example, Javey et al. increased the luminescence efficiency of MoS2 and WS2 monolayers by shielding the defects via chemical treatment.16,17 In addition, PL enhancement by surface plasmonic effects has also been realized in TMD monolayers coupled to noble metal nanostructures.18,19 While chemical treatments present the possibility to introduce contaminants into the monolayers, the high production cost of the complicated fabrication process of noble metal nanostructures restricts its practical application. Moreover, these methods result in global modification that affects the entire monolayers. With an interest to create miniaturized devices out of these monolayers, it is worthwhile to develop techniques that can engineer local modification on these monolayers with high spatial resolution. For example, one can make use of a standard masking technique
wo-dimensional (2D) transition-metal dichalcogenides (TMDs) with the formula of MX2, where M and X represent transition-metal and chalcogen atoms, respectively, can be viewed as the semiconducting counterpart of graphene in the 2D limit.1 Increasing research interest has been paid to TMDs due to their interesting properties, especially the indirect to direct band gap transition when the thickness of the material is thinned down from multilayer to monolayer.1−10 Among these TMDs, WS2, where W and S atoms are situated in a trigonal prismatic coordination sphere, has been demonstrated to be a promising building block for next-generation optoelectronics due to the interesting optical properties shown by its monolayer form. This is attributed to the sizable direct band gap (∼2.0 eV) of the WS2 monolayer.11 On the basis of the excellent properties, WS2 monolayers have been utilized in the demonstration of nanoelectronic and optoelectronic devices such as excitonic laser,12 valley-LED,13 FETs,14 and solar cells.15 While researchers are exploring the potential of WS 2 monolayers, it is soon realized that applications of WS2 monolayers are still restricted by the low quantum yield of the photoluminescence (PL). Especially the intrinsic chalcogen vacancies in the samples grown by chemical vapor deposition © 2016 American Chemical Society
Received: October 21, 2016 Accepted: December 29, 2016 Published: December 29, 2016 713
DOI: 10.1021/acsnano.6b07118 ACS Nano 2017, 11, 713−720
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Figure 1. Optical images and fluorescence property of as-grown products. (a) Bright-field OM image of pristine WS2 monolayer microflakes. (b) Raman spectrum of WS2 monolayers. (c) FM image of pristine WS2 monolayer microflakes under yellow light (550−580 nm) excitation. (d) PL spectrum of WS2 monolayers at room temperature captured under 532 nm laser excitation.
regions on the thick microflake. As a result, the potential of the CVD grown WS2 microflakes can be markedly enhanced.
to facilitate spatially defined modification in these monolayers. However, in the event where the monolayer microflakes are randomly scattered on the substrate, the use of mask for local modification becomes less practical, since the design of the mask needs to match the position of the microflakes. Thus, a straightforward way to further improve and modify the optical property of WS2 monolayers after synthesis is desired. In this report, we present a multipurpose and effective method to directly enhance the fluorescent emission of WS2 monolayers using a scanning focused laser beam. The WS2 monolayers were grown by CVD method, and they appeared as microflakes (microtriangles or microstars) randomly scattered on a Si substrate. The microflakes were subjected to raster treatments of a focused laser beam in ambient conditions. After the treatments, the monolayers were found to exhibit ∼9 times enhancement in the intensity of fluorescent emission. The enhancement arises from the healing of defects in WS2 monolayers via substitution of the sulfur vacancies with oxygen. Hence the laser beam treatment provides an effective handle to facilitate localized chemical modification of the WS2 monolayer and allows us to study the interplay between chemical composition of the monolayers and its fluorescence properties. Utilizing the high spatial resolution of a focused laser beam, the modification of the optical properties can be localized to micron-size domain. Consequently, on demand micropatterns with various optical properties could be engineered onto the monolayer microflakes. By carefully optimizing the laser powers, hidden information could be directly written into the WS2 monolayers, and thus microsteganography is achieved. This is realized by changing the fluorescence properties of the WS2 monolayers via laser modification while ensuring the morphology of the samples remained unchanged. Therefore, the written message can only be read under light excitation, where it reveals itself by way of contrast in fluorescence intensity. The same focused laser also facilitates on demand thinning of WS2 multilayers down to monolayers, turning them into fluorescence active components. With a scanning focused laser beam, micropatterns are readily created on WS2 multilayers through selective thinning of specific
RESULTS AND DISCUSSIONS WS2 monolayers are characterized by their triangular shape. Figure 1a shows small- and medium-sized triangular-shaped monolayers with uniform appearance and a distinct light-blue color under bright-field OM imaging with a mercury light source. Here, monolayers with a base length of
