Letter pubs.acs.org/NanoLett
Fluorescence Concentric Triangles: A Case of Chemical Heterogeneity in WS2 Atomic Monolayer Hongwei Liu,†,§ Junpeng Lu,*,†,‡ Kenneth Ho,† Zhenliang Hu,† Zhiya Dang,∥ Alexandra Carvalho,*,‡ Hui Ru Tan,§ Eng Soon Tok,† and Chorng Haur Sow*,†,‡ †
Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 Center For Advanced 2D Materials and Graphene Research Center, National University of Singapore, 6 Science Drive 2, Singapore 117546 § Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, 08-03, Singapore 138634 ∥ Department of Nanochemistry, Istituto Italiano di Tecnologia, Via Morego 30, 16163, Genova, Italy ‡
S Supporting Information *
ABSTRACT: We report a novel optical property in WS2 monolayer. The monolayer naturally exhibits beautiful in-plane periodical and lateral homojunctions by way of alternate dark and bright band in the fluorescence images of these monolayers. The interface between different fluorescence species within the sample is distinct and sharp. This gives rise to intriguing concentric triangular fluorescence patterns in the monolayer. The novel optical property of this special WS2 monolayer is facilitated by chemical heterogeneity. The photoluminescence of the bright band is dominated by emissions from trion and biexciton while the emission from defect-bound exciton dominates the photoluminescence at the dark band. The discovery of such concentric fluorescence patterns represents a potentially new form of optoelectronic or photonic functionality. KEYWORDS: 2D materials, transitional metal dichalcogenides, fluorescence, homojunction, heterogeneity alternate dark and bright band in fluorescence emission. This gives rise to a beautiful triangular WS2 monolayer with concentric fluorescence triangles. The fluorescence patterning is facilitated by the lateral homojunctions in the monolayer directly formed by a one-step growth process. We believe that such lateral homojunctions are formed by monolayers of WS2 which are either S-rich or S-poor. More specifically, the bright regions correspond to regions which are S-rich while the dark regions correspond to regions which are S-poor. To the best of our knowledge, this is the first report of such fluorescent concentric triangles in as grown 2D TMDs. The discovery of such concentric fluorescence patterns is exciting as it represents a potentially new form of optoelectronic/photonic functionality. Among the TMDs with formulas MX2, where M is a transition metal and X is a chalcogenides, WS2 is one of the most promising materials due to its excellent optical properties shown in monolayer form11,12 as demonstrated by the realization of an excitonic laser based on monolayer WS2.13 Besides having a sizable direct band gap (∼2.0 eV),14 monolayer WS2 is nearly transparent and displays high mechanical strength and great flexibility.15 These properties
H
eterojunctions consisting of two dissimilar crystalline semiconductors with different bandgaps represent exciting and fascinating systems which have captured significant attention. A heterojunction is an essential building-block in electronic and optoelectronic devices of high-speed transistors,1 solar cells2 and light-emitting diodes.3 Recently, lateral heterojunctions between two analogous two-dimensional (2D) transitional metal dichalcogenides (TMDs) with different direct bandgaps have been realized.4−7 This invention opens a new avenue to engineer their optical properties and this can certainly lead to potential photonic applications.8 However, heterojunctions possess inherent challenges in fabrication due to the lattice mismatches over the boundaries and differences in thermal expansion.9,10 While a lot of research efforts have been devoted to the development of heterojunction comprising two different types of TMDs, it would significantly simplify the operation process of 2D photonics if the heterojunction could be realized in a homogeneous TMD by way of engineering the optical properties within the 2D TMD itself. Therefore, lateral homojunction in a TMD of single material is desired. Here we demonstrate that an in-plane periodical and lateral homojunction with sharp interface can be formed within a WS2 monolayer. By way of chemical vapor deposition growth, these WS2 monolayers are typically grown with the shape of equilateral triangular microflake on the surface of the substrate. Such lateral homojunctions reveal themselves by way of © XXXX American Chemical Society
Received: May 25, 2016 Revised: July 26, 2016
A
DOI: 10.1021/acs.nanolett.6b02111 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Under UV light excitation, only the edges of the flake presents weak fluorescence (Figure S1c). This is attributed to the temporary suppression of the direct exciton recombination at band nesting resonance energy region.16,17 In TMDs monolayers, the relaxation channels of photocarriers are excitation energy dependent.17 Hence, the fluorescence becomes stronger with decreasing excitation energy. Under yellow light excitation (Figure S1f), the WS2 monolayer presents brightest red color fluorescence because the energy of the light is closer to the resonance of the exciton energy. Almost the entire area of the flake is fluorescence active, but the emission of the central region is weaker than at the edges. This is consistent with previous observations.14,18 Given the intense brightness of the emitted fluorescence, the subsequent FM images shown from here on are those captured under yellow light excitation. The distinctly intense fluorescent emission along the edge is clear evidence of heterogeneity within the triangular WS2 monolayer. Such heterogeneity cannot be detected via OM nor AFM in topography mode, suggesting that chemical and/or structural heterogeneity could be at play. Gutiérrez et al.14 presented similar observation of the enhanced edge fluorescence in WS2 monolayer and they attributed this to the edge structure and chemistry. Cong et al.18 reported similar observation of the suppression of PL intensity of the center region as compared to the edge. They suggested that the suppression is related to the existence of the structural and charged defects, such as S-vacancy as these defects were presumed to contribute to n-type doping of the WS2 monolayer.19,20 In contrast, Kim et al.21 observed the enhanced fluorescence at the edges and grain boundaries in WS2 monolayers. They assigned this to the high concentration of S vacancies in the edge region. Taking a closer look at Figure 1c, one can see that in addition to the bright edge fluorescence, the emission brightness of the central portion of the triangle is also very significant. Clearly, the red fluorescent triangle is divided into three smaller and equal triangles by the grain boundaries. This is illustrated by the schematic inset in Figure 1c where the three dark lines mark the grain boundaries inside the WS2 monolayer. Figure S2 shows zoom-in images of the center region of the WS2 monolayer obtained using AFM imaging. Figure S2a shows the height topography image while Figure S2b shows the phase contrast image. The grain boundaries are directly shown in both the AFM images with greater contrast appearing in the case of phase image. The grain boundaries in a WS2 monolayer represent a region with greater degree of mismatch, defects, vacancies, and so forth and these factors can weaken and shift the PL emission, which was also observed in other TMDs monolayers.22,23 Interestingly, when the triangular WS2 monolayer becomes larger, the fluorescence pattern of the WS2 monolayers becomes more and more complex. Figure 2 shows the images obtained from the as-grown WS2 microflakes captured via different microscopic techniques. OM image (Figure 2a) shows that the surface of the substrate is peppered with quite a number of triangular WS2 microflakes of various sizes. Inset in Figure 2a shows a zoom-in view of a large WS2 microflake with a base length of ∼120 μm. Figure 2b shows SEM image of a large triangular WS2 monolayer. Surprising results were obtained when we started examining the larger microflakes by comparing the OM and FM images of these WS2 monolayers. For ease of comparison, Figure 1b,c are reproduced here as
confer monolayer WS2 the potential to meet the demands of next generation optoelectronic devices requiring materials to be transparent for interactive displays, robust for wide distribution and flexible for wearable devices. Before realizing these practical applications, good control over the optical properties within WS2 monolayers is important. The creation of fluorescence patterns, demonstrated in this paper, is a useful method to modulate the optical properties of these monolayers. The WS2 monolayers grown on the silica/silicon substrate were characterized using optical microscope (OM), scanning electron microscope (SEM), fluorescence microscope (FM), atomic force microscope (AFM), Raman spectroscopy (RS) and Photoluminescence (PL). Isolated flakes with regular triangular shape, sharp edges, and clean surface are clearly observed on the silica/silicon substrate (Figure 1a,b). The dimensions of the triangular flakes cover a wide range with the length of the edges from a few tens up to hundreds of micrometers.
Figure 1. (a) AFM image of an as-grown WS2 monolayer. The inset shows the line profile along the white line. The measured thickness of the monolayer is ∼0.7 nm. (b) Bright-field and (c) FM images of a typical WS2 monolayer with the length of the edge ∼20 μm. Inset shows the schematic of FM emission of the triangular monolayer with grain boundaries. Scale bar =5 μm.
Figure 1a−c shows a series of microscopic images obtained from a typical small triangular WS2 microflake. In this work, any microflake with a base length that is
