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Fluctuation imaging of disorder in monolayer semiconductors

Tom T. C. Sistermans, Rasmus H. Godiksen, Sara A. Elrafei, Alberto G. Curto

Abstract

Monolayer semiconductors hold great potential for nanoscale electronics, optoelectronics, and photonics. Excitons dominate their optical properties. As their electric fields extend outside the monolayer, they are sensitive to their surroundings. Thus, disorder can cause exciton instability, which is detrimental to device performance and critical for scalability and reproducibility. Here, we adapt a super-resolution fluorescence fluctuation microscopy technique to image localized exciton fluctuations in monolayer semiconductors, allowing us to identify unstable spots in an otherwise continuous monolayer with constant fluorescence. These spots correspond to interfacial disorder measured by atomic force microscopy. We examine how different material interfaces influence the fluctuations by comparing several substrates and provide additional insight into the disorder behind the fluctuations using hyperspectral imaging. We also assess the reduction of disorder upon thermal annealing, evidenced by a decrease in fluctuations. Our results show that fluorescence fluctuation imaging can detect disorder features similar to those of atomic force microscopy and hyperspectral imaging, while being faster and easier to implement. Therefore, it is a promising method for evaluating the quality of monolayer semiconductors, particularly when integrated with nanostructures and heterostructures found in nano-optoelectronic devices.

Fluctuation imaging of disorder in monolayer semiconductors

Abstract

Monolayer semiconductors hold great potential for nanoscale electronics, optoelectronics, and photonics. Excitons dominate their optical properties. As their electric fields extend outside the monolayer, they are sensitive to their surroundings. Thus, disorder can cause exciton instability, which is detrimental to device performance and critical for scalability and reproducibility. Here, we adapt a super-resolution fluorescence fluctuation microscopy technique to image localized exciton fluctuations in monolayer semiconductors, allowing us to identify unstable spots in an otherwise continuous monolayer with constant fluorescence. These spots correspond to interfacial disorder measured by atomic force microscopy. We examine how different material interfaces influence the fluctuations by comparing several substrates and provide additional insight into the disorder behind the fluctuations using hyperspectral imaging. We also assess the reduction of disorder upon thermal annealing, evidenced by a decrease in fluctuations. Our results show that fluorescence fluctuation imaging can detect disorder features similar to those of atomic force microscopy and hyperspectral imaging, while being faster and easier to implement. Therefore, it is a promising method for evaluating the quality of monolayer semiconductors, particularly when integrated with nanostructures and heterostructures found in nano-optoelectronic devices.
Paper Structure (10 sections, 2 equations, 6 figures)

This paper contains 10 sections, 2 equations, 6 figures.

Table of Contents

  1. Introduction
  2. Super-resolved optical fluctuation imaging
  3. Imaging disorder in semiconductor monolayers
  4. Fluctuations for characterizing monolayer quality
  5. Conclusions
  6. Methods

Figures (6)

  • Figure 1: Optical fluctuation imaging for monolayer semiconductors. Schematic video frames of a two-dimensional emitter with fluctuating bright spots on a constant background. Average fluorescence intensity and linearized SOFI images calculated from the illustrative frames, which show a strong suppression of background fluorescence and an increased visibility of the fluctuating spots.
  • Figure 2: Localized and independent fluorescence fluctuations for two points on a WS$_2$ monolayer. Left: measured time traces showing distinct fluorescence fluctuations for two points on the same monolayer on an ITO substrate, separated by 4 $\mu$m, compared to a stable fluorescent dye. Right: autocorrelation of the time traces of points 1 and 2 and cross-correlation between them as a function of time delay, proving that the localized fluctuations are independent despite being on the same monolayer. Both points are located on the sample in Figure \ref{['main:fig3_intro_qSOFI']}.
  • Figure 3: Fluctuation imaging reveals disorder in a WS$_2$ monolayer. (a) Sources of disorder in a monolayer and their effects. (b) Comparison between height, time-averaged fluorescence, and qSOFI for a monolayer WS$_2$ on an ITO-coated substrate. (c) Cross-section along the dashed line in (b) comparing the height profile and the qSOFI value.
  • Figure 4: Mapping disorder in a WS$_2$ monolayer on hBN through changes in exciton properties. Top: height measured by AFM, time-averaged fluorescence, and fluorescence fluctuations shown as qSOFI. Bottom: exciton emission peak energy, linewidth, and trion intensity relative to neutral excitons retrieved from hyperspectral images of the same areas. Arrows point to two disordered areas showing opposing trends in relative fluorescence intensity, fluctuation strengths, and trion-to-neutral-exciton intensity ratios.
  • Figure 5: Effect of annealing on disorder for a WS$_2$ monolayer on an hBN substrate. Top and bottom rows: same monolayer before and after annealing. Small wrinkles disappear while some bubbles combine to form larger ones.
  • ...and 1 more figures