Achieving extremely high optical contrast of atomically-thin MoS2

Achieving High Visibility of Monolayer MoS2 Using Backside-Illuminated Thin Films

Interference reflection microscopy (IRM) and backside absorbing layer microscopy (BALM) have emerged as powerful optical microscopy methods for the study of nanomaterials and biological samples. These techniques consist in using an inverted optical microscope in reflection mode to observe objects deposited either on glass (IRM) or on a nanometric absorbing metallic film (BALM). The thickness of the BALM absorbing layer and of optional additional transparent layers, as well as the choice of incident wavelength and top medium, act as powerful levers for maximizing the resultant contrast of a given sample. However, the use of BALM to study samples with high absorption coefficient has been limited so far in the literature. Furthermore, the complex refractive index (ñ = n + iκ) of layers in a specific BALM optical stack have so far not been measured, and thus experimentally informed simulations are nonexistent. In this work, we use variable angle spectroscopic ellipsometry (VASE) to measure ñ(λ) of each layer in BALM (and IRM) stacks consisting of different combinations of glass, Cr, Au, and AlOx, with the two-dimensional (2D) transition metal dichalcogenide (TMD) MoS2 as the sample under study. Using the measured values, we simulate contrast spectra and compare them against data. We manipulate the film thicknesses and top media to engineer favorable contrast conditions both in the blue and red regimes of the visible spectrum, with peak contrasts of ≈80%. Finally, we demonstrate how the 2-layer BALM stack can double as both an optically sensitive system and at the same time a metal-oxide-semiconductor structure allowing for electrostatic gating. We use this structure to do in situ local charge density imaging of a 2D MoS2 capacitor, which showcases the versatility of these types of BALM stacks.

Identification of exfoliated monolayer hexagonal boron nitride films with a digital color camera under white light illumination

Optical microscopy with white light illumination has been employed when obtaining exfoliated monolayer hexagonal boron nitride (1L hBN) films from a large number of randomly placed films on a substrate. However, real-time observation of 1L hBN using a color camera under white light illumination remains challenging since hBN is transparent in the visible wavelength range. The poor optical constant of 1L hBN films in microphotographs is significantly improved using a Si substrate coated with a SiNxthin-film (SiNx/Si). When observing hBN thin films on SiNx/Si using a color digital camera in an optical microscope under white light illumination, the clarity of the captured color images depends on the thickness of the SiNxfilm (d). For real-time direct observation, thedwas optimized based on quantitative chromatic studies tailored to Bayer filters of a color image sensor. Through image simulation, it was determined that the color difference between 1L hBN and the bare substrate is maximized atd= 59 or 70 nm, which was experimentally verified. The SiNx/Si with optimizeddvalues visualized 1L hBN films without requiring significant contrast enhancement via image processing under white light illumination in real-time. Furthermore, the captured color photographs facilitate the reliable determination of the number of layers in few-layer hBN films using the contrast of the green channel of the images.

Ultrafast laser ablation, intrinsic threshold, and nanopatterning of monolayer molybdenum disulfide

Laser direct writing is an attractive method for patterning 2D materials without contamination. Literature shows that the ultrafast ablation threshold of graphene across substrates varies by an order of magnitude. Some attribute it to the thermal coupling to the substrates, but it remains by and large an open question. For the first time the effect of substrates on the femtosecond ablation of 2D materials is studied using MoS₂ as an example. We show unambiguously that femtosecond ablation of MoS₂ is an adiabatic process with negligible heat transfer to the substrates. The observed threshold variation is due to the etalon effect which was not identified before for the laser ablation of 2D materials. Subsequently, an intrinsic ablation threshold is proposed as a true threshold parameter for 2D materials. Additionally, we demonstrate for the first time femtosecond laser patterning of monolayer MoS₂ with sub-micron resolution and mm/s speed. Moreover, engineered substrates are shown to enhance the ablation efficiency, enabling patterning with low-power ultrafast oscillators. Finally, a zero-thickness approximation is introduced to predict the field enhancement with simple analytical expressions. Our work clarifies the role of substrates on ablation and firmly establishes ultrafast laser ablation as a viable route to pattern 2D materials.

Optical Inspection of 2D Materials: From Mechanical Exfoliation to Wafer-Scale Growth and Beyond

Optical inspection is a rapid and non-destructive method for characterizing the properties of two-dimensional (2D) materials. With the aid of optical inspection, in situ and scalable monitoring of the properties of 2D materials can be implemented industrially to advance the development and progress of 2D material-based devices toward mass production. This review discusses the optical inspection techniques that are available to characterize various 2D materials, including graphene, transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), group-III monochalcogenides, black phosphorus (BP), and group-IV monochalcogenides. First, the authors provide an introduction to these 2D materials and the processes commonly used for their fabrication. Then they review several of the important structural properties of 2D materials, and discuss how to characterize them using appropriate optical inspection tools. The authors also describe the challenges and opportunities faced when applying optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques available for largely and precisely enhancing the optical signals from 2D materials. This comprehensive review of the current status and perspective of future trends for optical inspection of the structural properties of 2D materials will facilitate the development of next-generation 2D material-based devices.

Visualization of a hexagonal born nitride monolayer on an ultra-thin gold film via reflected light microscopy

Hexagonal boron nitride (h-BN) is an important insulating layered material for two-dimensional heterostructure devices. Among many applications, few-layer h-BN films have been employed as superior tunneling barrier films. However, it is difficult to construct a heterostructure with ultra-thin h-BN owing to the poor visibility of flakes on substrates, especially on a metallic surface substrate. Since reflectance from a metallic surface is generally high, a h-BN film on a metallic surface does not largely influence reflection spectra. In the present study, a thin Au layer with a thickness of ∼10 nm deposited on a Si substrate with a thermally grown SiO₂was used for visualizing h-BN flakes. The thin Au layer possesses conductivity and transparency. Thus, the Au/SiO₂/Si structure serves as an electrode and contributes to the visualization of an ultra-thin film according to optical interference. As a demonstration, the wavelength-dependent contrast of exfoliated few-layer h-BN flakes on the substrate was investigated under a quasi-monochromatic light using an optical microscope. A monolayer h-BN film was recognized in the image taken by a standard digital camera using a narrow band-pass filter of 490 nm, providing maximum contrast. Since the contrast increases linearly with the number of layers, the appropriate number of layers is identified from the contrast. Furthermore, the insulating property of a h-BN flake is examined using a conductive atomic force microscope to confirm whether the thin Au layer serves as an electrode. The tunneling current through the h-BN flake is consistent with the number of layers estimated from the contrast.

Single-walled carbon nanotube membranes as non-reflective substrates for nanophotonic applications

We demonstrate that single-walled carbon nanotube (SWCNT) membranes can be successfully utilized as nanometer-thick substrates for enhanced visualization and facilitated study of individual nanoparticles. As model objects, we transfer optically resonant 200 nm silicon nanoparticles onto pristine and ethanol-densified SWCNT membranes by the femtosecond laser printing method. We image nanoparticles by scanning electron and bright-field optical microscopy, and characterize by linear and Raman scattering spectroscopy. The use of a pristine SWCNT membrane allows to achieve an order-of-magnitude enhancement of the optical contrast of the nanoparticle bright field image over the results shown in the case of the glass substrate use. The observed optical contrast enhancement is in agreement with the spectrophotometric measurements showing an extremely low specular reflectance of the pristine membrane (≤0.1%). Owing to the high transparency, negligibly small reflectance and thickness, SWCNT membranes offer a variety of perspective applications in nanophotonics, bioimaging and synchrotron radiation studies.

Strain and Charge Doping Fingerprints of the Strong Interaction between Monolayer MoS2 and Gold

Gold-mediated exfoliation of MoS₂ has recently attracted considerable interest. The strong interaction between MoS₂ and Au facilitates preferential production of centimeter-sized monolayer MoS₂ with near-unity yield and provides a heterostructure system noteworthy from a fundamental standpoint. However, little is known about the detailed nature of the MoS₂-Au interaction and its evolution with the MoS₂ thickness. Here, we identify the specific vibrational and binding energy fingerprints of this interaction using Raman and X-ray photoelectron spectroscopy, which indicate substantial strain and charge doping in monolayer MoS₂. Tip-enhanced Raman spectroscopy reveals heterogeneity of the MoS₂-Au interaction at the nanoscale, reflecting the spatial nonconformity between the two materials. Micro-Raman spectroscopy shows that this interaction is strongly affected by the roughness and cleanliness of the underlying Au. Our results elucidate the nature of the MoS₂-Au interaction and guide strain and charge doping engineering of MoS₂.