Single-molecule, single-electron, single-photon

Lead Research Organisation: University of Bath
Department Name: Physics

Abstract

Scanning tunnelling microscopy (STM) can be used to control matter on the atomic scale. Initially, this required direct manipulation of individual atoms or molecules with the tip of the STM directly above the target. This allowed for simple nanoscale structure to be produced, in which each molecule is individually arranged into the desired structure. However, this one-at-a-time limitation leads to a cap on the production of nanoscale components.
Nonlocal atomic manipulation, pioneered at the University of Bath, offers the potential for hundreds of parallel reactions to occur at one time. Therefore far more reasonable nanofabrication is theoretically possible. In this process charge carriers are repeatedly injected from the STM tip into a surface causing remote molecules, up to tens of nanometers away from the tip, to be manipulated. Currently the final state of nonlocal atomic manipulation can be described by a two-dimensional diffusive model, however the exact configuration of the surface is not known.
The aims of this project are to investigate the theorised photo-emission from the junction of the STM that accompanies the nonlocal effect. Measurement of the spectrum of this emitted light could potentially lead to determination of the final electronic state of the injected charge carriers. This deeper understanding can lead to the ability to engineer properties of the nonlocal effects, such as the distance over which the effects occur and, hence, quantum coherent properties.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509589/1 01/10/2016 30/09/2021
1789382 Studentship EP/N509589/1 01/10/2016 30/06/2020 Henry Etheridge
 
Title Dataset for "Common source of light emission and nonlocal molecular manipulation on the Si(111)-7x7 surface" 
Description This dataset contains data supporting the results presented in the paper "Common source of light emission and nonlocal molecular manipulation on the Si(111)-7x7 surface". It includes the data used to plot each figure associated with this publication, together with the raw oscilloscope data in .csv format. The study combines the results of the two near identical experimental techniques - nonlocal atomic manipulation and light emission from a scanning tunnelling microscope - for the system of toluene molecules chemisorbed on the Si(111)-7×7 surface at room temperature. The radial dependence of molecular desorption away from the tip injection site conforms to a two-step ballistic-diffusive transport of the injected hot electrons across the surface, with a threshold bias voltage of +2.0 V. We find the same threshold voltage of +2.0 V for light emission from the bare Si(111)-7×7 surface. Comparing these results with previous published spectra we propose that both the manipulation and the light emission follow the same hot electron dynamics, only differing in the outcome of the final relaxation step which may result in either molecular manipulation, or photon emission. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
 
Title Distribution of bromobenzene or toluene after negative bias nonlocal STM injection with varying injection duration and voltage on Si(111)-7x7 
Description Database consists of 50 nm before and after injection scans of a Si(111)-7x7 surface dosed with bromobenzene or toluene. The injections are 900 pA pulses of duration between 1 and 500 s, at either -1.6 V or -2.1 V which result in manipulation of the chemisorped phenyl-group molecules in a radius surrounding the tip-site. Prior to injection, drift correction has been applied to reduce the relative movement of the tip and the surface. The tip is positioned 10 nm from the corner of the before image to maximise the scanned radial distance from the tip site. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? No  
Impact From this database the radial distribution of manipulated bromobenzene and therefore the distribution of charge carriers during injection has been estimated as a function of the number of injected holes at two differing injection biases. From this several key properties have been measured, including the charge carrier diffusion length which has possibly been observed to vary with injection duration, and the probability of manipulation per charge carrier. The radius at which half of all molecules have been manipulated has been calculated and compared to the charge carrier inflation-diffusion model and a plasmon mediated manipulation model, and is found to be consistent with the former model. This outcome allows the transport of hot charge carriers on Si(111)-7x7 to be more well understood, providing evidence of the inflation-diffusion model. 
 
Title Estimation of the half-manipulation radius for inflation diffusion and plasmon mediated manipulation 
Description The expected radius at which half of the adsorbed molecules will have been desorped during nonlocal injection of charge carriers on a Si(111)-7x7 surface has been calculated based on three different injection-manipulation models; 2D diffusion of injected charge carriers from a point source, 2D diffusion of injected charge carriers from a top-hat distribution, and injected charge carrier decay into plasmon mediated manipulation. 
Type Of Material Computer model/algorithm 
Year Produced 2018 
Provided To Others? No  
Impact This model compares three different possible methods by which a hot charge carrier could induce desorption of chemisorped molecules on Si(111)-7x7, resulting in different predictions of the half manipulation radius as a function of the number of injected charge carriers. With a sufficiently large number of injected charge carriers, the half-manipulation radius for both the point source and top-hat models is expected to be approximately proportional to the log of the number of injected charge carriers. Whereas the plamson-mediated model predicts it to be proportional to the root of the log of the number of injected carriers. The top-hat model also predicts a plateau region for short injections; for injections below a threshold number of injected charge carriers the half manipulation radius is expected to vary only slightly. In comparison with data, both the short duration plateau region and the long duration log relationship are observed. This suggests that the manipulation is mediated by a charge carriers after undergoing inflation and 2D diffusion. Thereby suggesting the nonlocal manipulation is preceeded by charge carrier transport not plasmon excitation. 
 
Title Model for estimating the nonlocal branching ratio 
Description The branching ratio is a measurement of the ratio of two differing outcomes, here referring to whether a chemisorped molecule desorps or diffuses upon charge carrier injection. In local injections, such that the STM tip is positioned directly above the considered molecule, it has been previously measured to be independent of most injection parameters (e.g. injection bias, current). This new model expands the ability to measure the branching ratio to nonlocal injections, for which many molecules are manipulated simultaneously and each individual molecule cannot be tracked. Instead this ratio can be estimated by considering a simple compartmental model, noting that diffused molecules can be repeatedly manipulated whilst desorped molecules cannot, and considering only the final state of each molecule post-injection. This result allows the nonlocal-branching ratio to be calculated from either the difference in the distribution of the number of desorped and not-manipulated molecules, or equivalently the maximum proportion of diffused molecules. 
Type Of Material Computer model/algorithm 
Year Produced 2018 
Provided To Others? No  
Impact By comparing the local and nonlocal branching ratios, the manipulation mechanism can be further elucidated. For local injections the branching ratio has been used to show that the final state before injection is independent of the injection parameters, suggesting charge carriers decay to a common state immediately after injection from which they induce manipulation. It is also known that the nonlocal transport of injected charge carriers are transported in discrete surface states prior to manipulation. Assuming that the final manipulation step of both local and nonlocal manipulation is equivalent, then the branching ratio should also be the same; with similar independence to injection parameters.