robust molecular probe for Ångstrom-scale analytics in liquids

Abstract

Traditionally, nanomaterial profiling using a single-molecule-terminated scanning probe is performed at the vacuum–solid interface often at a few Kelvin, but is not a notion immediately associated with liquid–solid interface at room temperature. Here, using a scanning tunnelling probe functionalized with a single C60 molecule stabilized in a high-density liquid, we resolve low-dimensional surface defects, atomic interfaces and capture Ångstrom-level bond-length variations in single-layer graphene and MoS2. Atom-by-atom controllable imaging contrast is demonstrated at room temperature and the electronic structure of the C60–metal probe complex within the encompassing liquid molecules is clarified using density functional theory. Our findings demonstrates that operating a robust single-molecular probe is not restricted to ultra-high vacuum and cryogenic settings. Hence the scope of high-precision analytics can be extended towards resolving sub-molecular features of organic elements and gauging ambient compatibility of emerging layered materials with atomic-scale sensitivity under experimentally less stringent conditions.

Introduction

Sensing objects below the resolution limit of the eye began with the advent of water-based microscopes in 2000 BCE. Following a series of ground-breaking advances, instrument development eventually led to the modern electron and scanning probe microscopes which have not only advanced resolution capability but have enabled new research disciplines from quantum cryptology to single-molecular nanotechnology. Recently, high-resolution transmission electron microscopy1, scanning tunnelling microscopy (STM)2,3,4 and scanning tunnelling spectroscopy5 have made possible the analysis of a new class of ultra-thin materials. The atomic-scale structure, extrinsic doping, bonding states and chemical composition can be precisely measured for single-atom-thick electronic materials from the two-dimensional (2D) form of carbon, graphene6 to more recent transition metal dichalcogenides (TMDs)7 with potential for triggering a new wave of 2D nanodevice technologies.

However, preserving the surface integrity of single-atom-thick materials outside of ultra-high vacuum (UHV) settings, and extracting information with highest possible resolution, where each atom is directly exposed to contaminants is not trivial. Yet the payoff is immense. To take one illustrative example, it would allow rapid fingerprinting of a vast body of theoretically predicted 2D layered materials8 and simultaneously verify the ambient compatibility of such novel materials9 and 2D patterned structures10. A known alternative to vacuum to protect air-sensitive surfaces, is the liquid–solid interface in which a STM can be operated. This field of research has an almost 30-year history, from early reports on achieving atomic resolution on solid surfaces immersed in water11, liquid nitrogen12 and acidic solution13environments, observation of molecular dynamics14, decoding molecular layer-underlying surface epitaxial relationship15, capturing oxidation catalysis reactions16 and in investigating through high-resolution STM images the supramolecular chemistry of molecules17and pattern formation during molecular self-assembly18 at the liquid–solid electrical interface with a non-functionalized metal probe. More recently it has been demonstrated that by terminating the apex of a scanning probe (including STM and AFM) with a single molecule, it is possible to further the limits of spatial resolution and enhance chemical contrast of the low-dimensional materials under study19,20,21,22,23,24,25,26. However, these single-molecule-terminated scanning probes have been mainly demonstrated to operate at UHV in a temperature scale ranging from 4 to 100 K (refs 20, 21, 22, 25, 27, 28). The protocols for engineering such molecular probes, in particular bonding effects between the molecule and the metal-apex, have been documented in detail22,24,27,28,29,30. Conversely, the benefits of the molecular probe have not been fully exploited under standard laboratory conditions, mostly because of instabilities in molecule–metal coupling at room temperature. In the present work, we demonstrate that by engineering and operating a single-molecule-terminated Au STM probe in high-density liquids, it is possible to control random fluctuations of the molecule at the metal apex, maintain a clean interface protected from ambient contaminants thereby resulting in a robust single-molecular probe with prolonged lifetime.

Results

Electron tunnelling in a high-density liquid environment

The central concept of our experiment is depicted in Fig. 1, a metallic (Au) STM probe functionalized with a C60 molecule is brought close to the surface (epitaxial graphene on SiC31) held in a Teflon-based liquid cell filled with silicone oil ([-Si(CH3)2O)-]n). C60 geometry permits easier lateral manipulation and transfer to the tip from the surface at ambient conditions than, for example, CO, and the protocol for C60 transfer is well established19,22,24,32. Single C60-terminated STM tips are routinely used to probe organic materials and C60 clusters anchored on metal STM tips have also been demonstrated previously, to investigate low-dimensional surface defects33. Such molecular cluster-based tips are relatively robust (loss of a single molecule at the apex can be compensated by its nearest neighbour) but interpretation of the observed chemical contrast improvements is not trivial. The mechanism of electron tunnelling from the C60-terminated Au probe through a non-polar liquid medium into the conductive graphene layer may be modelled as tunnelling through a vacuum gap where the tunnelling current (Itunnel) is exponentially dependent on the distance (Z) between the probe and the conductive surface using an effective energy barrier height (φeff) as shown below,

Figure 1: Description of the liquid-based STM experimental design.

Schematic of the liquid-cell setup with the single-molecule-terminated Au tip (connected to an external current pre-amplifier circuit) positioned over an epitaxial graphene sample (on which the bias voltage is applied). The entire liquid cell holds an electrochemically inert and high-density silicone oil at room-temperature conditions. The liquid cell is based on Teflon.