Research Fellow

PhD in Chemistry, University of Bern, Switzerland (2018)

I work on the EPSRC funded Quantum-Interference Enhanced Thermoelectrics (QUIET) project, studying thermal transport in SPM junctions of single molecules and molecular complexes.

1. Fast Data Sorting with Modified Principal Component Analysis to Distinguish Unique Single Molecular Break Junction Trajectories
   J. M. Hamill, X. T. Zhao, G. Mészáros, M. R. Bryce, and M. Arenz.
   Physical Review Letters, 120(1), , 2018
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   A simple and fast analysis method to sort large data sets into groups with shared distinguishing characteristics is described and applied to single molecular break junction conductance versus electrode displacement data. The method, based on principal component analysis, successfully sorts data sets based on the projection of the data onto the first or second principal component of the correlation matrix without the need to assert any specific hypothesis about the expected features within the data. This is an improvement on the current correlation matrix analysis approach because it sorts data automatically, making it more objective and less time consuming, and our method is applicable to a wide range of multivariate data sets. Here the method is demonstrated on two systems. First, it is demonstrated on mixtures of two molecules with identical anchor groups and similar lengths, but either a π (high conductance) or a σ (low conductance) bridge. The mixed data are automatically sorted into two groups containing one molecule or the other. Second, it is demonstrated on break junction data measured with the π bridged molecule alone. Again, the method distinguishes between two groups. These groups are tentatively assigned to different geometries of the molecule in the junction. © 2018 American Physical Society.

2. Single-molecule detection of dihydroazulene photo-thermal reaction using break junction technique
   C. Huang, M. Jevric, A. Borges, S. T. Olsen, J. M. Hamill, J. -T. Zheng, Y. Yang, A. Rudnev, M. Baghernejad, P. Broekmann, A. U. Petersen, T. Wandlowski, K. V. Mikkelsen, G. C. Solomon, M. Brøndsted Nielsen, and W. Hong.
   Nature Communications, 8, , 2017
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   Charge transport by tunnelling is one of the most ubiquitous elementary processes in nature. Small structural changes in a molecular junction can lead to significant difference in the single-molecule electronic properties, offering a tremendous opportunity to examine a reaction on the single-molecule scale by monitoring the conductance changes. Here, we explore the potential of the single-molecule break junction technique in the detection of photo-thermal reaction processes of a photochromic dihydroazulene/vinylheptafulvene system. Statistical analysis of the break junction experiments provides a quantitative approach for probing the reaction kinetics and reversibility, including the occurrence of isomerization during the reaction. The product ratios observed when switching the system in the junction does not follow those observed in solution studies (both experiment and theory), suggesting that the junction environment was perturbing the process significantly. This study opens the possibility of using nano-structured environments like molecular junctions to tailor product ratios in chemical reactions.

3. Electrochemical control of the single molecule conductance of a conjugated bis(pyrrolo)tetrathiafulvalene based molecular switch
   L. J. O’Driscoll, J. M. Hamill, I. Grace, B. W. Nielsen, E. Almutib, Y. Fu, W. Hong, C. J. Lambert, and J. O. Jeppesen.
   Chemical Science, 8(9), 6123-6130, 2017
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   As the field of unimolecular electronics develops, there is growing interest in the development of functionalised molecular wires, such as switches, which will allow for more complex molecular-scale circuits. To this end, a three redox state single molecule switch, 1, based on bis(pyrrolo)tetrathiafulvalene (BPTTF) has been designed, synthesised and investigated using scanning tunnelling microscopy break junction (STM-BJ) studies and quantum transport calculations. Oxidising the BPTTF unit increases its conjugation, which was anticipated to increase the molecular conductance of 1. By changing the redox state of 1 electrochemically it was possible to vary the single molecule conductance by more than an order of magnitude (from 10-5.2G0 to 10-3.8G0). Simulations afforded a qualitatively similar trend. An additional, higher conductance feature is present in most traces at junction sizes of around 2.0 nm-further extension affords the switchable lower conductance feature at junction sizes closer to the molecular length (ca. 3.0 nm). Analysis of the conductance traces shows that these two conductance features occur sequentially in nearly all junctions. This behaviour is attributed to an alternative initial junction conformation in which one or more of the BPTTF sulfur atoms acts as an anchoring group. This hypothesis is supported by a computational study of binding conformations and STM-BJ studies on a model compound, 2, with only one thiol anchor. Our results indicate that the redox properties of BPTTF make it an excellent candidate for use in single molecule switches. © 2017 The Royal Society of Chemistry.

4. Molecular rectifier composed of DNA with high rectification ratio enabled by intercalation
   C. Guo, K. Wang, E. Zerah-Harush, J. Hamill, B. Wang, Y. Dubi, and B. Xu.
   Nature Chemistry, 8(5), 484-490, 2016
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   The predictability, diversity and programmability of DNA make it a leading candidate for the design of functional electronic devices that use single molecules, yet its electron transport properties have not been fully elucidated. This is primarily because of a poor understanding of how the structure of DNA determines its electron transport. Here, we demonstrate a DNA-based molecular rectifier constructed by site-specific intercalation of small molecules (coralyne) into a custom-designed 11-base-pair DNA duplex. Measured current-voltage curves of the DNA-coralyne molecular junction show unexpectedly large rectification with a rectification ratio of about 15 at 1.1 V, a counter-intuitive finding considering the seemingly symmetrical molecular structure of the junction. A non-equilibrium Green’s function-based model – parameterized by density functional theory calculations – revealed that the coralyne-induced spatial asymmetry in the electron state distribution caused the observed rectification. This inherent asymmetry leads to changes in the coupling of the molecular HOMO-1 level to the electrodes when an external voltage is applied, resulting in an asymmetric change in transmission. © 2016 Macmillan Publishers Limited. All rights reserved.

5. B. Xu, K. Wang, J. Hamill, and R. Colvard. Modulate and control of detailed electron transport of single molecule. , 2016.
6. Three-State Single-Molecule Naphthalenediimide Switch: Integration of a Pendant Redox Unit for Conductance Tuning
   Y. Li, M. Baghernejad, A. -G. Qusiy, D. Zsolt Manrique, G. Zhang, J. Hamill, Y. Fu, P. Broekmann, W. Hong, T. Wandlowski, D. Zhang, and C. Lambert.
   Angewandte Chemie – International Edition, 54(46), 13586-13589, 2015
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   We studied charge transport through core-substituted naphthalenediimide (NDI) single-molecule junctions using the electrochemical STM-based break-junction technique in combination with DFT calculations. Conductance switching among three well-defined states was demonstrated by electrochemically controlling the redox state of the pendent diimide unit of the molecule in an ionic liquid. The electrical conductances of the dianion and neutral states differ by more than one order of magnitude. The potential-dependence of the charge-transport characteristics of the NDI molecules was confirmed by DFT calculations, which account for electrochemical double-layer effects on the conductance of the NDI junctions. This study suggests that integration of a pendant redox unit with strong coupling to a molecular backbone enables the tuning of charge transport through single-molecule devices by controlling their redox states. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

7. Mapping the details of contact effect of modulated Au-octanedithiol-Au break junction by force-conductance cross-correlation
   K. Wang, J. M. Hamill, J. Zhou, and B. Xu.
   Journal of the American Chemical Society, 136(50), 17406-17409, 2014
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   We have measured the force and conductance of Au-octanedithiol-Au junctions using a modified conducting atomic force microscopy break junction technique with sawtooth modulations. Force-conductance two-dimensional cross-correlation histogram (FC-2DCCH) analysis for the single-molecule plateaus is demonstrated. Interestingly, four strong correlated regions appear in FC-2DCCHs consistently when modulations with different amplitudes are applied, in sharp contrast to the results under no modulation. These regions reflect the conductance and force changes during the transition of two molecule/electrode contact configurations. As the modulation amplitude increases, intermediate transition states of the contact configurations are discerned and further confirmed by comparing individual traces. This study unravels the relation between force and conductance hidden in the data of a modulated single-molecule break junction system and provides a fresh understanding of electron transport properties at molecule/electrode interfaces. © 2014 American Chemical Society.

8. Measurement and understanding of single-molecule break junction rectification caused by asymmetric contacts
   K. Wang, J. Zhou, J. M. Hamill, and B. Xu.
   Journal of Chemical Physics, 141(5), , 2014
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   The contact effects of single-molecule break junctions on rectification behaviors were experimentally explored by a systematic control of anchoring groups of 1,4-disubstituted benzene molecular junctions. Single-molecule conductance and I-V characteristic measurements reveal a strong correlation between rectifying effects and the asymmetry in contacts. Analysis using energy band models and I-V calculations suggested that the rectification behavior is mainly caused by asymmetric coupling strengths at the two contact interfaces. Fitting of the rectification ratio by a modified Simmons model we developed suggests asymmetry in potential drop across the asymmetric anchoring groups as the mechanism of rectifying I-V behavior. This study provides direct experimental evidence and sheds light on the mechanisms of rectification behavior induced simply by contact asymmetry, which serves as an aid to interpret future single-molecule electronic behavior involved with asymmetric contact conformation. © 2014 AIP Publishing LLC.

9. Force and conductance molecular break junctions with time series crosscorrelation
   J. M. Hamill, K. Wang, and B. Xu.
   Nanoscale, 6(11), 5657-5661, 2014
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   Force and conductance, measured across 4,4′-bipyridine simultaneously, are crosscorrelated using a two dimensional (2D) histogram method. The result is a 2D multivariate statistical analysis superior to current one dimensional histogram techniques for exploring significant conductance and force modulations within SMBJs. This method is sensitive enough to crosscorrelate signal modulations between force and conductance traces associated with contact geometry perturbations predicted in literature such as Au-molecule contact twisting and slipping during junction elongation. © 2014 the Partner Organisations.

10. Measurement and control of detailed electronic properties in a single molecule break junction
    K. Wang, J. Hamill, J. Zhou, C. Guo, and B. Xu.
    Faraday Discussions, 174, 91-104, 2014
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    The lack of detailed experimental controls has been one of the major obstacles hindering progress in molecular electronics. While large fluctuations have been occurring in the experimental data, specific details, related mechanisms, and data analysis techniques are in high demand to promote our physical understanding at the single-molecule level. A series of modulations we recently developed, based on traditional scanning probe microscopy break junctions (SPMBJs), have helped to discover significant properties in detail which are hidden in the contact interfaces of a single-molecule break junction (SMBJ). For example, in the past we have shown that the correlated force and conductance changes under the saw tooth modulation and stretch-hold mode of PZT movement revealed inherent differences in the contact geometries of a molecular junction. In this paper, using a bias-modulated SPMBJ and utilizing emerging data analysis techniques, we report on the measurement of the altered alignment of the HOMO of benzene molecules with changing the anchoring group which coupled the molecule to metal electrodes. Further calculations based on Landauer fitting and transition voltage spectroscopy (TVS) demonstrated the effects of modulated bias on the location of the frontier molecular orbitals. Understanding the alignment of the molecular orbitals with the Fermi level of the electrodes is essential for understanding the behaviour of SMBJs and for the future design of more complex devices. With these modulations and analysis techniques, fruitful information has been found about the nature of the metal-molecule junction, providing us insightful clues towards the next step for in-depth study. © The Royal Society of Chemistry 2014.

11. Structure determined charge transport in single DNA molecule break junctions
    K. Wang, J. M. Hamill, B. Wang, C. Guo, S. Jiang, Z. Huang, and B. Xu.
    Chemical Science, 5(9), 3425-3431, 2014
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    Experimental study of the charge transport properties associated with structural variations due to a change in the ionic environment will provide essential physical information in determining the nature of DNA molecules. This work reports an experimental study of the change in electronic transport properties induced by the conformational transition of a poly d(GC)4 DNA. By gradually increasing the concentration of MgCl2 in the buffer solution from 0 M to 4 M, the conductance of the single DNA molecule decreased by two orders of magnitude. Circular dichroism (CD) measurements confirmed that a B to Z conformational transition caused the reduction in conductance. Using a stretch-hold mode scanning probe microscopy break junction (SPMBJ) technique, this B-Z transition process was monitored and a transition trend line was successfully achieved from conductance measurements alone. The transition midpoint occurred at a MgCl2 concentration of 0.93 M for this DNA sequence. This method provides a general tool to study transitions of molecular properties associated with conductance differences. © 2014 the Partner Organisations.