Imaging atomically thin films using neutral atom microscopy Geetika Bhardwaj, Krishna Rani Sahoo, Rahul Sharma, Parswa Nath, and Pranav R. Shirhatti ∗ Tata Institute of Fundamental Research Hyderabad, 36/P Gopanpally, Hyderabad 500046, Telangana, India Imaging surfaces using low energy neutral atom scattering is a relatively recent development in the field of microscopy. In this work, using collimated beams of He and Kr atoms as an incident probe, we demonstrate that thin MoS2 films up to a single monolayer on SiO2/Si substrate can be imaged. MoS2 films were prepared using chemical vapour deposition method and their thickness was characterized by Raman spectroscopy. Atom scattering based maps show that specular scattered He and Kr flux from MoS2 films is consistently larger than that from underlying SiO2/Si substrate. Measurements using Kr atoms at relatively larger energies than He, reveal the role of change in local surface roughness in contrast generation. I. INTRODUCTION Developments in microscopy and imaging techniques have made a profound impact on our understanding of a wide range of physical phenomena. The variety of techniques developed over the years is impressive – such as the light microscope [1], sub- ˚ A level resolution elec- tron microscopes [2, 3], surface probe techniques such as atomic force and scanning tunneling microscopes [4– 7], to name a few. A relatively recent addition to this gamut of microscopy techniques is based on scattering of low energy neutral atoms from surfaces and is referred to as Neutral Atom Microscopy (NAM) / Scanning Helium Microscopy (SHeM), when He atoms are used as incident probe [8–10]. NAM offers several unique advantages and is a promising addition to the existing toolkit. In this regard, two key features are worth highlighting: Firstly, kinetic energy of incident neutral atoms used as probe, typically lie in the range of ten to a few hundred meV. Being at least an order of magnitude smaller than typ- ical chemical bond energies (∼ few eV), this acts as a soft and non-destructive probe [11, 12]. It is particularly well suited for imaging delicate surface structures other- wise susceptible to beam induced damage. Secondly, it is an universally applicable technique using which a wide range of surfaces including conducting, insulating, mag- netic, optically transparent, to name a few, can be probed directly without any elaborate sample preparation. An interesting area that is yet to be fully explored is the applicability of NAM methods for imaging thin films, especially those made up of two dimensional (2D) mate- rials. Previously reported atom scattering experiments with 2D materials such as graphene grown on Pt, Ru have provided information about graphene - substrate interac- tion and phonon structure of graphene overlayer [13–15]. These surfaces also exhibit high reflectivity, about 20% for He atoms. Furthermore, films of 2D materials are rel- atively chemically inert. Hence, the requirement of elab- ∗ Author to whom correspondence should be addressed. e-mail: pranavrs@tifrh.res.in orate in-situ sample preparation and cleaning, in order to prepare a well defined surface is less stringent. These fea- tures make them interesting candidates for atomic mir- rors to be used for focusing atomic beams. Given the highly surface specific nature of atom scat- tering process and being a soft and non-destructive probe, NAM is well suited for studying thin film struc- tures like 2D materials. This work focuses on two main questions, namely, whether an atomically thin film of 2D material and its underlying substrate can give rise to a measurable contrast in NAM, and further to understand its nature. Here, contrast (C ) is defined as the normal- ized difference in signals I 1 and I 2 observed from two different surface features and is expressed as [10]: C =     I 1 - I 2 I 1 + I 2     (1) In this work, using our recently developed pinhole col- limation based NAM apparatus, we explore its capability for imaging thin MoS 2 films grown on SiO 2 /Si substrate. Based on a detailed comparison of NAM images obtained using beams of He and Kr atoms, optical microscopy in conjunction with Raman spectroscopy for sample charac- terization, we demonstrate that thin films of MoS 2 , up to a single monolayer can be imaged using NAM. Further, incidence energy dependence of the observed contrasts for He and Kr show distinctly different trends. We dis- cuss the possible contrast generating mechanisms at play based on these observations. II. EXPERIMENTAL DETAILS A. Instrument setup Design of our NAM apparatus is based on producing a collimated atomic beam using a series of pinholes [9]. This beam scatters from the sample surface mounted on a movable platform. The scattered atoms are detected by a stagnation (flux) detector as a function of sample arXiv:2105.09777v2 [physics.chem-ph] 20 Jul 2021