Dr Marco Sacchi


Royal Society University Research Fellow
Fellow of The Higher Education Academy
+44 (0)1483 686834
03 AZ 02

Biography

Areas of specialism

Quantum Biology; Computational Chemistry; Density Functional Theory; Surface Science; Materials Modelling; Science Policy

University roles and responsibilities

  • Chair of the Early Career Researchers' Forum of the University Doctoral College
  • Early Career Researcher Representative on the University Research & Innovation Committee
  • Energy and Materials Theme's Coordinator in the School of Chemistry and Chemical Engineering

    Research

    Research interests

    Research projects

    My publications

    Highlights

    Tamtögl, A.; Bahn, E.; Sacchi, M.; Zhu, J.; Ward, D. J.; Jardine, A. P.; Jenkins, S. J.; Fouquet, P.; Ellis, J.; Allison, W., Motion of water monomers reveals a kinetic barrier to ice nucleation on graphene. Nature Communications 2021, 12 (1), 3120.

    Tamtögl, A.; Sacchi, M.; Avidor, N.; Calvo-Almazán, I.; Townsend, P. S. M.; Bremholm, M.; Hofmann, P.; Ellis, J.; Allison, W., Nanoscopic diffusion of water on a topological insulator. Nature Communications 2020, 11 (1), 278.

    https://doi.org/10.1038/s41467-019-14064-7

    Publications

    MARCO SACCHI, (2021)Motion of water monomers reveals a kinetic barrier to ice nucleation on graphene., In: Nature communications. Nature Research

    The interfacial behaviour of water remains a central question to fields as diverse as protein folding, friction and ice formation 1,2. While the structural and dynamical properties of water at interfaces differ strongly from those in the bulk, major gaps in our knowledge at the molecular level still prevent us from understanding these ubiquitous chemical processes. Information concerning the microscopic motion of water comes mostly from computational simulation 3,4 but the dynamics of molecules, on the atomic scale, is largely unexplored by experiment. Here we present experimental results combined with ab initio calculations to provide a detailed insight into the behaviour of water monomers on a graphene surface. We show that motion occurs by activated hopping on the graphene lattice. The dynamics of water diffusion displays remarkably strong signatures of cooperative behaviour due to repulsive forces between the monomers. The repulsive forces enhance the monomer lifetime (tm ≈ 3 s at TS = 125 K) in a free-gas phase that precedes the nucleation of ice islands and, in turn, provides the opportunity for our experiments to be performed. Our results give a unique molecular perspective on a kinetic barrier to ice nucleation on a crystalline surface, providing new understanding of the processes involved in ice formation. I ce often forms easily on solid surfaces and to understand why 1 that happens, the molecular basis of the water-surface inter-2 action needs to be studied 2,5. The structure, dynamics and 3 chemical properties of water at interfaces differ from those of bulk 4 water and ice 6–8. The early stages of ice nucleation involve ex-5 ceedingly small time and length scales 9 and while ice nucleation 6 and phase transitions are well understood macroscopically, unrav-7 elling the microscopic details presents one of the great challenges 8 in physical sciences with important implications from the chem-9 istry of the Earth's atmosphere 10 to physicochemical processes oc-10 curring on cosmic dust grains 11. 11 It is the motion of water molecules at surfaces, that controls 12 these fundamental phenomena in physics, chemistry and biology 13 as well as a diverse range of technological processes 1,2,12. Wet-14 ting, hydrophobicity and ice nucleation are all very widely studied 15 on the macroscopic scale, using routine methods such as contact 16 angle measurements 13–15. However, more precise measurements, 17 with a molecular level of detail, are much scarcer, despite the fact 18 that an understanding could open up new opportunities for the 19 design of advanced materials, by exploiting our ability to tune 20 surfaces at the nanoscale 16. For example, ice nucleation on sur-21 faces is alone of huge technological relevance to fields as diverse 22 as wind power 13,17 , aviation 14,18 and telecommunications 13. 23 Water is fundamentally challenging to study with atomic reso-24 lution. It is difficult to achieve sufficient contrast and resolution 25 with imaging techniques 19 , particularly in order to understand 26 the position of the H atoms and thus the molecular orientation. 27 Electron based techniques such as LEED also scatter weakly from 28 hydrogen and present a severe risk of damage, in the form of wa-29 ter dissociation 20,21. Some structural studies of water have been 30 possible experimentally, but are usually restricted to flat metal 31 surfaces 20–25 or a few ionic crystals, such as NaCl 19,26. These 32 studies have revealed the role of short range attractive forces. 33 Dynamics and low coverage measurements, which could exam-34 ine the nature of water interactions more generally, are further 35 complicated by fast diffusion rates and the short lifetimes of wa-36 ter monomers. Insight has therefore been mostly limited to that 37 possible with numerical simulations 3,4 , often without any direct 38 experimental validation to support them. 39 In this paper we report the serendipitous discovery of a regime 40 where freely mobile water can be studied on a Ni(111) supported 41 graphene surface. We use the helium spin-echo (HeSE) technique, 42 illustrated in Figure 1a, to measure surface correlations in the 43 water monomer motion (see Methods). HeSE uses wavepacket 44 splitting and recombination to give temporal sensitivity over pi-45 cosecond timescales, resulting in data of the form shown in the 46 inset of Figure 1a. The use of these very low energy He atoms 47 completely excludes any possibility of damage or dissociation of 48 the water. As we describe below, by analysing the dephasing rates 49 in the correlation measurements to obtain the signatures shown 50 in Figure 1b, we are able to establish that, contrary to expecta-51 tions, strong repulsive interactions exist between adsorbed water 52 molecules. We attribute these forces to dipolar interactions, aris-53 ing from structural hindrance of water reorientation by the ad-54 sorption geometry. The repulsion leads to a kinetic barrier that 55 inhibits the nucleation of solid ice, while extending the surface 56 lifetime of water monomers and simultaneously making our mea-57 surements possible. 58 Results 59 In order to identify the range of conditions where individual wa-60 ter molecules are mobile, we carried out extensive adsorption 61 and desorption measurements on the graphene/Ni(111) surface. 62 The substrate was prepared in ultra-high vacuum (UHV) condi-63 tions and graphene was grown using established methods 27 (see 64 Methods and Sample preparation in the supplementary informa-65 tion (SI)). Growing a thick film of water at 100 K results in a 66 1

    L Slocombe, J. S Al-Khalili, M Sacchi (2021)Quantum and classical effects in DNA point mutations: Watson–Crick tautomerism in AT and GC base pairs, In: Physical chemistry chemical physics : PCCP23(7)pp. 4141-4150

    Proton transfer along the hydrogen bonds of DNA can lead to the creation of short-lived, but biologically relevant point mutations that can further lead to gene mutation and, potentially, cancer. In this work, the energy landscape of the canonical A–T and G–C base pairs (standard, amino–keto) to tautomeric A*–T* and G*–C* (non-standard, imino–enol) Watson–Crick DNA base pairs is modelled with density functional theory and machine-learning nudge-elastic band methods. We calculate the energy barriers and tunnelling rates of hydrogen transfer between and within each base monomer (A, T, G and C). We show that the role of tunnelling in A–T tautomerisation is statistically unlikely due to the presence of a small reverse reaction barrier. On the contrary, the thermal populations of the G*–C* point mutation could be non-trivial and propagate through the replisome. For the direct intramolecular transfer, the reaction is hindered by a substantial energy barrier. However, our calculations indicate that tautomeric bases in their monomeric form have remarkably long lifetimes.

    Jianping Yang, Minhan Li, Yuanyuan Ma, Jun Chen, Wei Luo, Marco Sacchi, Wan Jiang, Robert Lawrence (2021)Residual Chlorine Induced Cationic Active Species on Porous Cu Electrocatalyst for Highly Stable Electrochemical CO2 Reduction to C2, In: Angewandte Chemie Wiley

    Electrochemical carbon dioxide (CO2) reduction reaction (CO2RR) is an attractive approach to deal with the excessive emission of CO2 and to produce valuable fuels and chemicals in a carbon-neutral way. Many efforts have been devoted to boost the activity and selectivity of high-value multicarbon products (C2+) on Cu-based electrocatalysts. However, Cu-based CO2RR electrocatalysts suffer from poor catalytic stability mainly due to the structural degradation and loss of active species under CO2RR condition. To date, most reported Cu-based electrocatalysts present stabilities over dozens of hours, which limits the advance of Cu-based electrocatalysts for CO2RR. Here, a porous chlorine-doped Cu electrocatalyst is reported and exhibits high C2+ Faradaic efficiency (FE) of 53.8 % at-1.00 V versus reversible hydrogen electrode (VRHE). Importantly, the catalyst exhibited an outstanding catalytic stability in long-term electrocatalysis over 240 hours. Experimental results show that the chlorine-induced stable cationic Cu 0-Cu + species and the well-preserved structure with abundant active sites are found to be critical to maintain the high FE of C2+ in the long-term run of electrochemical CO2 reduction.

    Youngchan Kim, Federico Bertagna, Edeline M. D’Souza, Derren J. Heyes, Linus O. Johannissen, Eveliny T. Nery, Antonio Pantelias, Alejandro Sanchez-Pedreño Jimenez, Louie Slocombe, Michael G. Spencer, Jim Al-Khalili, Gregory S. Engel, Sam Hay, Suzanne M. Hingley-Wilson, Kamalan Jeevaratnam, Alex R. Jones, Daniel R. Kattnig, Rebecca Lewis, Marco Sacchi, Nigel S. Scrutton, S. Ravi P. Silva, Johnjoe McFadden (2021)Quantum Biology: An Update and Perspective, In: Quantum Reports3(6)pp. 80-126 MDPI AG

    Understanding the rules of life is one of the most important scientific endeavours and has revolutionised both biology and biotechnology. Remarkable advances in observation techniques allow us to investigate a broad range of complex and dynamic biological processes in which living systems could exploit quantum behaviour to enhance and regulate biological functions. Recent evidence suggests that these non-trivial quantum mechanical effects may play a crucial role in maintaining the non-equilibrium state of biomolecular systems. Quantum biology is the study of such quantum aspects of living systems. In this review, we summarise the latest progress in quantum biology, including the areas of enzyme-catalysed reactions, photosynthesis, spin-dependent reactions, DNA, fluorescent proteins, and ion channels. Many of these results are expected to be fundamental building blocks towards understanding the rules of life.

    Jonathan Davidson, MARCO SACCHI, Fabrice Gorrec, Stuart Clarke, Stephen Jenkins (2021)Halogen Bonding in Bicomponent Monolayers: Self-Assembly of a Homologous Series of Iodinated Perfluoroalkanes with Bipyridine, In: Langmuir

    A homologous series of halogen bonding monolayers based on terminally iodinated perfluoroalkanes and 4,4′-bipyridine have been observed on a graphitic surface and noninvasively probed using powder X-ray diffraction. An excellent agreement is observed between the X-ray structures and density functional theory calculations with dispersion force corrections. Theoretical analysis of the binding energies of the structures indicate that these halogen bonds are strong (25 kJ mol–1), indicating that the layers are highly stable. The monolayer structures are found to be distinct from any plane of the corresponding bulk structures, with limited evidence of partitioning of hydrocarbon and perfluoro tectons. The interchain interactions are found to be slightly stronger than those in related aromatic systems, with important implications for 2D crystal engineering.

    Arjun Raghavan, Louie Slocombe, Alexander Spreinat, David J Ward, William Allison, John Ellis, Andrew P Jardine, Marco Sacchi, Nadav Avidor (2020)Alkali metal adsorption on metal surfaces: new insights from new tools, In: Physical chemistry chemical physics : PCCP

    The adsorption of sodium on Ru(0001) is studied using 3He spin-echo spectroscopy (HeSE), molecular dynamics simulations (MD) and density functional theory (DFT). In the multi-layer regime, an analysis of helium reflectivity, gives an electron-phonon coupling constant of λ = 0.64 ± 0.06. At sub-monolayer coverage, DFT calculations show that the preferred adsorption site changes from hollow site to top site as the supercell increases and the effective coverage, θ, is reduced from 0.25 to 0.0625 adsorbates per substrate atom. Energy barriers and adsorption geometries taken from DFT are used in molecular dynamics calculations to generate simulated data sets for comparison with measurements. We introduce a new Bayesian method of analysis that compares measurement and model directly, without assuming analytic lineshapes. The value of adsorbate-substrate energy exchange rate (friction) in the MD simulation is the sole variable parameter. Experimental data at a coverage θ = 0.028 compares well with the low-coverage DFT result, giving an effective activation barrier Eeff = 46 ± 4 meV with a friction γ = 0.3 ps-1. Better fits to the data can be achieved by including additional variable parameters, but in all cases, the mechanism of diffusion is predominantly on a Bravais lattice, suggesting a single adsorption site in the unit cell, despite the close packed geometry.

    I Calvo-Almazán, Marco Sacchi, A Tamtögl, E Bahn, MM Koza, S Miret-Artés, P Fouquet (2016)Ballistic Diffusion in Poly-aromatic Hydrocarbons on Graphite, In: Journal of Physical Chemistry Letters7pp. 5285-5290 American Chemical Society

    This work presents an experimental picture of molecular ballistic diffusion on a surface, a process which is difficult to pinpoint since it generally occurs at very short length scales. By combining neutron-time-of-flight data, with molecular dynamics simulations and density functional theory calculations, we provide a complete description of the ballistic translations and rotations of a poly-aromatic hydrocarbon (PAH) adsorbed on the basal plane of graphite. Pyrene, C16H10, adsorbed on graphite is a unique system where at relative surface coverages of about 10-20 %, its mean free path matches the experimentally accessible time/space scale of neutron time-of-flight spectroscopy (IN6 at the Institut Laue-Langevin). The comparison between the diffusive behavior of large and small PAHs such as pyrene and benzene adsorbed on graphite, brings a strong experimental indication that the interaction between molecules is the dominating mechanism in the surface diffusion of poly-aromatic hydrocarbons adsorbed on graphite.

    DC Madden, I Temprano, M Sacchi, M Blanco-Rey, SJ Jenkins, SM Driver (2014)Self-Organized Overlayers Formed by Alanine on Cu{311} Surfaces, In: Journal of Physical Chemistry C118(32)pp. 18589-18603 American Chemical Society

    Chirality can manifest itself in diverse ways when a molecule adsorbs on a metal surface. A clear understanding of the interplay between molecular chirality, “footprint chirality”, and chirality in the long-range self-organization is crucial if metal surfaces are to be exploited for enantioselective heterogeneous catalysis or enantio-discriminating sensors. We have investigated the self-organization of l-alanine adsorbed as alaninate on Cu{311}, using reflection–absorption infrared spectroscopy in conjunction with first-principles calculations to determine bonding configurations, and low-energy electron diffraction and scanning tunnelling microscopy to elucidate structural features. Three ordered structures are seen. One has a symmetric lattice and 3-point adsorbate bonding (the “symmetric lattice” or SL phase); the others, occurring at higher coverage, have chiral lattices and also involve 2-point bonding (the “chiral lattice” or CL phase). Possible models for these structures are discussed, together with the roles of footprint chirality and of long-range chirality in the self-organization. These results set the forms of chirality seen in alaninate overlayers on Cu{110} and {100} surfaces into a wider context. The common underlying principles should help in establishing a general framework for understanding the behavior of chiral adsorbates on low-symmetry metal surfaces.

    R Bisson, M Sacchi, RD Beck (2010)Mode-specific reactivity of CH4 on Pt(110)-(1 x 2) : The concerted role of stretch and bend excitation, In: Physical Review B82 The American Physical Society

    The state-resolved reaction probability of CH4 on Pt(110)−(1×2) was measured as a function of CH4 translational energy for four vibrational eigenstates comprising different amounts of C-H stretch and bend excitation. Mode-specific reactivity is observed both between states from different polyads and between isoenergetic states belonging to the same polyad of CH4. For the stretch/bend combination states, the vibrational efficacy of reaction activation is observed to be higher than for either pure C-H stretching or pure bending states, demonstrating a concerted role of stretch and bend excitation in C-H bond scission. This concerted role, reflected by the nonadditivity of the vibrational efficacies, is consistent with transition state structures found by ab initio calculations and indicates that current dynamical models of CH4 chemisorption neglect an important degree of freedom by including only C-H stretching motion.

    M Sacchi (2012)Mode-specificity and transition state-specific energy redistribution in the chemisorption of CH4 on Ni{100}, In: Physical Chemistry Chemical Physics14pp. 15879-15887 Royal Society of Chemistry

    We have investigated methane (CH4) dissociative chemisorption on the Ni{100} surface by first-principles molecular dynamics (MD) simulations. Our results show that this reaction is mode-specific, with the ν1 state being the most strongly coupled to efficient energy flow into the reaction coordinate when the molecule reaches the transition state. By performing MD simulations for two different transition state (TS) structures we provide evidence of TS structure-specific energy redistribution in methane chemisorption. Our results are compared with recently reported state-resolved measurement of methane adsorption probability on nickel surfaces, and we find that a strong correlation exists between the highest vibrational efficacy measured on Ni{100} for the ν1 state and the calculated highest fractional vibrational energy content in this mode.

    AY Brewer, M Sacchi, JE Parker, CL Truscott, SJ Jenkins, SM Clarke (2013)The crystalline structure of the phenazine overlayer physisorbed on a graphite surface, In: Molecular Physics: an international journal in the field of chemical physics111(24)pp. 3823-3830 Taylor & Francis

    The monolayer crystal structure of phenazine adsorbed on graphite is determined by a combination of synchrotron X-ray diffraction and DFT calculations. The molecules adopt a rectangular unit cell with lattice parameters a = 13.55 Å and b = 10.55 Å, which contains 2 molecules. The plane group of the unit cell is p2gg, and each molecule is essentially flat to the plane of the surface, with only a small amount of out-of-plane tilt. Density functional theory (DFT) calculations find a minimum energy structure with a unit cell which agrees within 7.5% with that deduced by diffraction. DFT including dispersion force corrections (DFT+D) calculations help to identify the nature of the intermolecular bonding. The overlayer interactions are principally van der Waals, with a smaller contribution from weak C-H···N hydrogen bonds. This behaviour is compared with that of 4,4′-bipyridyl.

    M Sacchi, SJ Jenkins (2014)Co-adsorption of water and glycine on Cu{110}, In: Physical Chemistry Chemical Physics16pp. 6101-6107 Royal Society of Chemistry

    Amino acids are some of the simplest biological molecules, yet they nevertheless manifest the ability to construct an incredibly complex variety of structures in which a delicate balance of intermolecular chemical forces drives the dynamics of self-recognition and assembly. Understanding the mechanism by which chiral structures are naturally synthesized is also extremely relevant to pharmaceutical and biochemical industries, in which enantioselectivity and enantiospecificity are vital factors in producing biologically compatible drugs. In this context, the adsorption of simple, naturally occurring amino acids on single crystal surfaces has become the playground for studying chiral self-assembly at the atomic scale and investigating pathways to enantioselective catalytic synthesis using a bottom-up approach. In particular, in the last two decades, several groups have dedicated a concerted effort to understand the formation of chiral self-assembled supramolecular networks of alanine, glycine and proline on Cu{110}, Cu{100}1 and Cu{111} surfaces. In the past, with few exceptions,1 the vast majority of the atomistic studies on supramolecular assembly of amino acids on metal surfaces have been conducted under UHV conditions. It is therefore one of the main challenges ahead of the surface-science community to attempt to bridge the gap between experiments conducted under “dry” vacuum conditions (in which the amino acids adsorb in the absence of a solvent and a co-adsorbate) and the more biologically and pharmaceutically relevant “wet” studies. In fact, when water is present in the system, a competition exists between the formation of hydrogen bonds between an amino acid with another and between an amino acid and the water shell immediately surrounding it. The interaction between amino acids and water is also particularly relevant to corrosion protection, since amino acids have recently become a natural and ecologically compatible alternative to traditional amine-based corrosion inhibitors. In this work we study the co-adsorption of water with glycine, the simplest naturally occurring amino acid, using first-principles density functional theory. Although in the past some authors have tried to account for the solvation of amino acids in the gas-phase, few studies have treated the solvation and interaction between adsorbed glycine and water molecules quantum mechanically.

    R Bisson, M Sacchi, RD Beck (2010)State-resolved reactivity of CH4 on Pt(110)-(1 x 2): The role of surface orientation and impact site, In: Journal of Chemical Physics132(9) American Institute of Physics

    The reactivity of methane (CH 4 ) on Pt(110)-(1×2) Pt(110)-(1×2) has been studied by quantum state-resolved surface reactivity measurements. Ground state reaction probabilities, S 0 (v=0)≅S 0 (laser-off) S0(v=0)≅S0(laser-off) , as well as state-resolved reaction probabilities S 0 (2ν 3 ) S0(2ν3) , for CH 4 CH4 excited to the first overtone of the antisymmetric C–H stretch (2ν 3 ) (2ν3) have been measured at incident translational energies in the range of 4–64 kJ/mol. We observe S 0 (2ν 3 ) S0(2ν3) to be up to three orders of magnitude higher than S 0 (v=0) S0(v=0) , demonstrating significant vibrational activation of CH 4 CH4 dissociation on Pt(110)-(1×2) Pt(110)-(1×2) by 2ν 3 2ν3 excitation. Furthermore, we explored the azimuthal and polar incident angle dependence of S 0 (2ν 3 ) S0(2ν3) and S 0 (v=0) S0(v=0) for a fixed incident translational energy E t =32 kJ/mol Et=32 kJ/mol . For incidence perpendicular to the missing row direction on Pt(110)-(1×2) Pt(110)-(1×2) and polar angles θ>40° θ>40° , shadowing effects prevent the incident CH 4 CH4 molecules from impinging into the trough sites. Comparison of this polar angle dependence with reactivity data for incidence parallel to the missing rows yields state-resolved site specific reactivity information consistent with a Pt(110)-(1×2) Pt(110)-(1×2) reactivity that is dominated by top layer Pt atoms located at the ridge sites. A comparison of S 0 (v=0) S0(v=0) measured on Pt(110)-(1×2) Pt(110)-(1×2) and Pt(111) yields a lower average barrier for Pt(110)-(1×2) Pt(110)-(1×2) by 13.7±2.0 kJ/mol 13.7±2.0 kJ/mol .

    M Sacchi, MCE Galbraith, SJ Jenkins (2012)The interaction of iron pyrite with oxygen, nitrogen and nitrogen oxides: a first-principles study, In: Physical Chemistry Chemical Physics14pp. 3627-3633 Royal Society of Chemistry

    Sulphide materials, in particular MoS2, have recently received great attention from the surface science community due to their extraordinary catalytic properties. Interestingly, the chemical activity of iron pyrite (FeS2) (the most common sulphide mineral on Earth), and in particular its potential for catalytic applications, has not been investigated so thoroughly. In this study, we use density functional theory (DFT) to investigate the surface interactions of fundamental atmospheric components such as oxygen and nitrogen, and we have explored the adsorption and dissociation of nitrogen monoxide (NO) and nitrogen dioxide (NO2) on the FeS2(100) surface. Our results show that both those environmentally important NOx species chemisorb on the surface Fe sites, while the S sites are basically unreactive for all the molecular species considered in this study and even prevent NO2 adsorption onto one of the non-equivalent Fe–Fe bridge sites of the (1 × 1)–FeS2(100) surface. From the calculated high barrier for NO and NO2 direct dissociation on this surface, we can deduce that both nitrogen oxides species are adsorbed molecularly on pyrite surfaces.

    BAJ Lechner, H Hedgeland, J Ellis, W Allison, M Sacchi, SJ Jenkins, BJ Hinch (2013)Quantum Influences in the Diffusive Motion of Pyrrole on Cu(111), In: Angewandte Chemie International Edition52(19)pp. 5085-5088

    Classical diffusion—quantum barrier: On Cu(111), pyrrole diffuses in channels, hopping between adjacent bridge sites over a barrier above hollow sites. The motion of the center of mass can be described classically; however, the activation barrier arises from the quantum character of internal vibrational modes that are largely unexcited during the motion. The unique helium spin‐echo experiment is indicated by the green sphere and arrows.

    T Zhang, M Sacchi, DA King, SM Driver (2012)Coverage-Dependent Structural Evolution in the Interaction of NO2 with Au{111}, In: Journal of Physical Chemistry C116(9)pp. 5637-5645 American Chemical Society

    We have used low-temperature STM, together with DFT calculations incorporating the effects of dispersion forces, to study from a structural point of view the interaction of NO2 with Au{111} surfaces. NO2 adsorbs molecularly on Au{111} at 80 K, initially as small, disordered clusters at the elbows of the type-x reconstruction lines of the clean-surface herringbone reconstruction, and then as larger, ordered islands on the fcc regions. Within the islands, the NO2 molecules define a (√3 × 2)rect. superlattice, for which we evaluate structural models. By around 0.25 ML coverage, the herringbone reconstruction has been lifted, accompanied by the formation of Au nanoclusters, and the islands have coalesced. At this stage, essentially the whole surface is covered with an overlayer consisting predominantly of domains of the (√3 × 2)rect. structure, but also containing less well-ordered regions. With further exposure, the degree of disorder in the overlayer increases; saturation occurs close to 0.43 ML.

    M Sacchi, DJ Wales, SJ Jenkins (2011)Mode-Specific Chemisorption of CH4 on Pt{110}-(1 x 2) Explored by First-Principles Molecular Dynamics, In: Journal of Physical Chemistry C115(44)pp. 21832-21842 American Chemical Society

    The chemisorption of CH4 on Pt{110}-(1 2) has been studied by vibrational analysis of the reaction pathway defined by the potential energy surface and, in time reversal, by first-principles molecular dynamics simulations of CH4 associative desorption, with the electronic structure treated explicitly using density functional theory. We find that the symmetric stretch vibration ν1 is strongly coupled to the reaction coordinate; our results therefore provide a firm theoretical basis for recently reported state-resolved reactivity measurements, which show that excitation of the ν1 normal mode is the most efficient way to enhance the reaction probability.

    BAJ Lechner, M Sacchi, AP Jardine, H Hedgeland, W Allison, J Ellis, SJ Jenkins, PC Dastoor, BJ Hinch (2013)Jumping, Rotating, and Flapping: The Atomic-Scale Motion of Thiophene on Cu(111), In: The Journal of Physical Chemistry Letters4(11)pp. 1953-1958 American Chemical Society

    Self-assembled monolayers of sulfur-containing heterocycles and linear oligomers containing thiophene groups have been widely employed in organic electronic applications. Here, we investigate the dynamics of isolated thiophene molecules on Cu(111) by combining helium spin-echo (HeSE) spectroscopy with density functional theory calculations. We show that the thiophene/Cu(111) system displays a rich array of aperiodic dynamical phenomena that include jump diffusion between adjacent atop sites over a 59–62 meV barrier and activated rotation around a sulfur–copper anchor, two processes that have been observed previously for related systems. In addition, we present experimental evidence for a new, weakly activated process, the flapping of the molecular ring. Repulsive inter-adsorbate interactions and an exceptionally high friction coefficient of 5 ± 2 ps–1 are also observed. These experiments demonstrate the versatility of the HeSE technique, and the quantitative information extracted in a detailed analysis provides an ideal benchmark for state-of-the-art theoretical techniques including nonlocal adsorbate–substrate interactions.

    M Sacchi, SJ Jenkins, H Hedgeland, AP Jardine, BJ Hinch (2011)Electronic Structure and Bonding of an Ionic Molecular Adsorbate: c-C5H5 on Cu{111}, In: Journal of Physical Chemistry C115(32)pp. 16134-16141 American Chemical Society

    Self-assembled monolayers containing conjugated π systems find application in organic electronics to functionalize and modify the electronic properties of metals and metal oxides. Isolated cyclopentadienyl is an aromatic molecular anion similar in size to benzene that, unlike benzene, adsorbs quite strongly even on coinage metal surfaces. In this study, the electronic structure, bonding, and minimum energy configuration of cyclopentadienyl (c-C5H5 or Cp) adsorbed on Cu{111} are calculated via first-principles density functional theory (DFT). The Cu{111} surface has been modeled within a (2√3 × 2√3)R30° cell, and the adsorbed Cp has been found to reside preferentially on the hollow sites, with a binding energy of 1.73 eV. Electronic population analysis reveals a net charge transfer of ∼1.1 electrons from the metal to the Cp, indicating that the adsorption is dominated by ionic bonding. The surface diffusion barrier between two adjacent hollow sites was calculated to be 55 meV, in good agreement with previously reported measurements by helium spin echo (HeSE) spectroscopy. It was found that lateral interactions do not significantly influence the binding energy and mobility of the adsorbate. The physical–chemical properties of this strongly bound but weakly mutually interacting molecular adsorbate suggest that Cp could become a model system for ionically adsorbed molecular adsorbates.

    Marco Sacchi, DJ Wales, SJ Jenkins (2017)Energy Landscapes and Dynamics of Glycine on Cu(110), In: Physical Chemistry Chemical Physics19(25)pp. 16600-16605 Royal Society of Chemistry

    Amino acids adsorbed over single crystal metal surfaces have emerged as prototypical systems for exploring the properties that govern the development of long-range chirality in self-assembled monolayers (SAM) and supramolecular 2D networks. In this study, we characterise the self-assembly mechanism for glycine on the Cu(110) surface. This process occurs on a time scale that is too fast for most atomically resolved microscopic techniques, so the mechanism we propose here provides new insight for an important unexplored surface phenomenon.

    H Hedgeland, BAJ Lechner, FE Tuddenham, AP Jardine, W Allison, J Ellis, M Sacchi, SJ Jenkins, BJ Hinch (2011)Weak Intermolecular Interactions in an Ionically Bound Molecular Adsorbate: Cyclopentadienyl/Cu(111), In: Physical Review Letters106(18) American Physical Society

    The dissociative adsorption of cyclopentadiene (C5H6) on Cu(111) yields a cyclopentadienyl (Cp) species with strongly anionic characteristics. The Cp potential energy surface and frictional coupling to the substrate are determined from measurements of dynamics of the molecule together with density functional calculations. The molecule is shown to occupy degenerate threefold adsorption sites and molecular motion is characterized by a low diffusional energy barrier of 40±3  meV with strong frictional dissipation. Repulsive dipole-dipole interactions are not detected despite charge transfer from substrate to adsorbate.

    M Sacchi, DJ Wales, SJ Jenkins (2012)Bond-selective energy redistribution in the chemisorption of CH3D and CD3H on Pt{1 1 0}-(1 × 2): A first-principles molecular dynamics study, In: Computational and Theoretical Chemistry990pp. 144-151 Elsevier

    We have investigated the chemisorption of CH3D and CD3H on Pt{1 1 0}-(1 × 2) by performing first-principles molecular dynamics simulations of the recombinative desorption of CH3D (from adsorbed methyl and deuterium) and of CD3H (from adsorbed trideuteromethyl and hydrogen). Vibrational analysis of the symmetry adapted internal coordinates of the desorbing molecules shows that excitation of the single C–D (C–H) bond in the parent molecule is strongly correlated with energy excess in the reaction coordinate. The results of the molecular dynamics simulations are consistent with observed mode- and bond-specific reactivity measurements for chemisorption of methane and its isotopomers on platinum and nickel surfaces.

    Anton Tamtögl, Marco Sacchi, Nadav Avidor, Irene Calvo-Almazán, Peter S. M. Townsend, Martin Bremholm, Philip Hofmann, John Ellis, William Allison (2020)Nanoscale measurement of water diffusion on a topological insulator: The origin of correlated motion and friction, In: Nature Communications11278 (2020) Nature Research

    The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water for instance in catalysis, biophysics and nanotribology is extremely challenging due to its ultrafast dynamics, and the complex interplay of intermolecular and molecule-surface interactions. Here we present the first experimental and computational study of the nanoscale-nanosecond motion of water at the surface of a topological insulator (TI, Bi2Te3). In addition to the technological relevance and scientific interest on the interfacial behaviour of water, understanding the interaction of TI surfaces with molecules is a key to design and manufacturing for future applications. However the surface chemistry of these materials has hitherto been hardly addressed and exploratory work on the motion of molecules on TI surfaces has been so far solely based on computational studies. By analysing the scattering lineshape from helium spinecho spectroscopy and comparing the results with van der Waals-corrected density functional theory calculations we are able to obtain a general insight into the diffusion and mobility of water on a topological insulator surface. Instead of the expected Brownian motion, we find strong evidence of a complex diffusion mechanism which follows an activated hopping motion on a corrugated potential energy surface and shows signatures of correlated motion with unusual repulsive interactions between the individual water molecules. From the experimental lineshape broadening we determine the diffusion coefficient, the diffusion energy and the pre-exponential factor.

    David Amabilino, Ioan Bâldea, Pol Besenius, Peter Beton, Matthew Blunt, Manfred Buck, Neil R. Champness, Lifeng Chi, Stuart Clarke, Giovanni Costantini, Steven De Feyter, Yuri Diaz Fernandez, Deepak Dwivedi, Karl-Heinz Ernst, Amar Flood, Brandon Hirsch, Robert Jones, Angelika Kühnle, Markus Lackinger, Trolle R. Linderoth, Natalia Martsinovich, Andrew Mount, Martin Nalbach, Claire-Marie Pradier, Talat Rahman, Rasmita Raval, Neil Robinson, Marco Sacchi, Sebastian Schwaminger, Steven L. Tait, Phil Woodruff, Han Zuilhof (2017)Supramolecular systems at liquid–solid interfaces: general discussion, In: Faraday Discussions204pp. 271-295 Royal Society of Chemistry
    Marco Sacchi, P Singh, H Hedgeland (2017)The Dynamics of Benzene on Cu(111): a Combined Helium Spin Echo and Dispersion-Corrected DFT Study into the Diffusion of Physisorbed Aromatics on Metal Surfaces, In: Faraday Discussions204pp. 471-485 Royal Society of Chemistry

    We use helium spin-echo spectroscopy (HeSE) to investigate the dynamics of the diffusion of benzene adsorbed on Cu(111). The results of these measurements show that benzene moves on the surface through an activated jump-diffusion process between adsorption sites on a Bravais lattice. Density Functional Theory (DFT) calculations with van der Waals (vdW) corrections help us understand that the molecule diffuses by jumps through non-degenerate hollow sites. The results of the calculations shed light on the nature of the binding interaction between this prototypical aromatic molecule and the metallic surface. The highly accurate HeSE experimental data provide a quantitatively stringent benchmark for the vdW correction schemes applied to the DFT calculations and we compare the performances of several dispersion interactions schemes.

    A Tamtogl, Marco Sacchi, I Calvo-Almazan, M Zbiri, MM Koza, WE Ernst, P Fouquet (2017)Ultrafast Molecular Transport on Carbon Surfaces: The Diffusion of Ammonia on Graphite, In: CARBON126pp. 23-30 Elsevier

    We present a combined experimental and theoretical study of the self-diffusion of ammonia on exfoliated graphite. Using neutron time-of- flight spectroscopy we are able to resolve the ultrafast diffusion process of adsorbed ammonia, NH3, on graphite. Together with van der Waals corrected density functional theory calculations we show that the diffusion of NH3 follows a hopping motion on a weakly corrugated potential energy surface with an activation energy of about 4 meV which is particularly low for this type of diffusive motion. The hopping motion includes further a significant number of long jumps and the diffusion constant of ammonia adsorbed on graphite is determined with D = 3.9.10-8 m2/s at 94K.

    David Amabilino, Ioan Bâldea, James Batteas, Pol Besenius, Peter Beton, Manfred Buck, Lifeng Chi, Giovanni Costantini, Philip Davies, Steven De Feyter, Yuri Diaz Fernandez, Deepak Dwivedi, Karl-Heinz Ernst, Amar Flood, Brandon Hirsch, Vincent Humblot, Robert Jones, Angelika Kühnle, Markus Lackinger, Nian Lin, Trolle R. Linderoth, Claire-Marie Pradier, Talat Rahman, Rasmita Raval, Neil Robinson, Marco Sacchi, Sebastian Schwaminger, Steven L. Tait, Phil Woodruff, Han Zuilhof (2017)Supramolecular effects in self-assembled monolayers: general discussion, In: Faraday Discussions204pp. 123-158 Royal Society of Chemistry
    SY Guo, SJ Jenkins, W Ji, Z Ning, C Polanyi, M Sacchi, C-G Wang (2015)Repulsion-Induced Surface-Migration, by Ballistics and Bounce, In: The Journal of Physical Chemistry Letters6(20)pp. 4093-4098 2015 American Chemical Society

    The motion of adsorbate molecules across surfaces is fundamental to self-assembly, material growth, and heterogeneous catalysis. Recent Scanning Tunneling Microscopy studies have demonstrated the electron-induced long-range surface-migration of ethylene, benzene, and related molecules, moving tens of Angstroms across Si(100). We present a model of the previously unexplained long-range recoil of chemisorbed ethylene across the surface of silicon. The molecular dynamics reveal two key elements for directed long-range migration: first ‘ballistic’ motion that causes the molecule to leave the ab initio slab of the surface traveling 3–8 Å above it out of range of its roughness, and thereafter skipping-stone ‘bounces’ that transport it further to the observed long distances. Using a previously tested Impulsive Two-State model, we predict comparable long-range recoil of atomic chlorine following electron-induced dissociation of chlorophenyl chemisorbed at Cu(110).

    M Sacchi, AY Brewer, SJ Jenkins, JE Parker, T Friščić, SM Clarke (2013)Combined Diffraction and Density Functional Theory Calculations of Halogen-Bonded Cocrystal Monolayers, In: Langmuir: the ACS journal of surfaces and colloids29(48)pp. 14903-14911

    This work describes the combined use of synchrotron X-ray diffraction and density functional theory (DFT) calculations to understand the cocrystal formation or phase separation in 2D monolayers capable of halogen bonding. The solid monolayer structure of 1,4-diiodobenzene (DIB) has been determined by X-ray synchrotron diffraction. The mixing behavior of DIB with 4,4′-bipyridyl (BPY) has also been studied and interestingly is found to phase-separate rather than form a cocrystal, as observed in the bulk. DFT calculations are used to establish the underlying origin of this interesting behavior. The DFT calculations are demonstrated to agree well with the recently proposed monolayer structure for the cocrystal of BPY and 1,4-diiodotetrafluorobenzene (DITFB) (the perfluorinated analogue of DIB), where halogen bonding has also been identified by diffraction. Here we have calculated an estimate of the halogen bond strength by DFT calculations for the DITFB/BPY cocrystal monolayer, which is found to be ∼20 kJ/mol. Computationally, we find that the nonfluorinated DIB and BPY are not expected to form a halogen-bonded cocrystal in a 2D layer; for this pair of species, phase separation of the components is calculated to be lower energy, in good agreement with the diffraction results.

    N García Rey, Marco Sacchi, SJ Jenkins, H Arnolds (2017)Dipole Moment Reversal in a Polar Organic Monolayer Probed by Sum and Difference Frequency Spectroscopy, In: The Journal of Physical Chemistry Part C: Nanomaterials and Interfaces121(12)pp. 6692-6700 American Chemical Society

    We investigate the adsorption of pyridine on Cu(110) in ultra-high vacuum with a combination of work function measurements and femtosecond infrared-visible sum and difference frequency generation (SFG/DFG). A monolayer of pyridine substantially reduces the work function by 2.9 eV due to the large pyridine dipole. We perform density functional theory (DFT) calculations that provide us with a dipole moment change upon adsorption in very good agreement with the experimental results. The pyridine dipole strongly enhances the sum frequency response of the surface electrons, but surprisingly reduces the surface difference frequency signal. We propose a model based on the static electric field-induced nonlinear optical response generated by the collective electric field of the adsorbate layer. The pyridine dipole switches direction from the ground to the excited electronic state, as charge moves from nitrogen to the ring. SFG can then be enhanced by the electric field of adsorbed pyridine in its ground electronic state, while the 2.33 eV incident photon in DFG excites electrons into the pyridine LUMO, which reverses the electric field in the adsorbate layer and reduces the nonlinear optical response. The model is verified by 2.33 eV pump – SFG probe spectroscopy, where the pump pulse is found to reduce the surface electron response on a subpicosecond timescale. This demonstrates the potential to manipulate the work function in organic electronic devices by photon-induced dipole moment reversal.

    Anton Tamtogl, Marco Sacchi, Nadav Avidor, Irene Calvo-Almazan, Peter Townsend, Martin Bremholm, Philip Hofmann, John Ellis, William Allison (2019)Nanoscopic diffusion of water on a topological insulator., In: Nature Communications11278 Nature Research

    The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water in fields such as catalysis, biophysics and nanotribology is challenging due to its ultrafast motion, and the complex interplay of inter-molecular and molecule-surface interactions. Here we present an experimental and computational study of the nanoscale-nanosecond motion of water at the surface of a topological insulator (TI), Bi2Te3. Understanding the chemistry and motion of molecules on TI surfaces, while considered a key to design and manufacturing for future applications, has hitherto been hardly addressed experimentally. By combining helium spin-echo spectroscopy and density functional theory calculations, we are able to obtain a general insight into the diffusion of water on Bi2Te3. Instead of Brownian motion, we find an activated jump diffusion mechanism. Signatures of correlated motion suggest unusual repulsive interactions between the water molecules. From the lineshape broadening we determine the diffusion coefficient, the diffusion energy and the pre-exponential factor.

    M Sacchi, AY Brewer, JE Parker, CL Truscott, SJ Jenkins, SM Clarke (2014)Supramolecular self-assembled network formation containing N-Br halogen bonds in physisorbed overlayers, In: Physical Chemistry Chemical Physics16pp. 19608-19617 Royal Society of Chemistry

    The formation of a halogen bonded self-assembled co-crystal physisorbed monolayer containing N⋯Br interactions is reported for the first time. The co-crystal monolayer is identified experimentally by synchrotron X-ray diffraction and the structure determined. Density functional theory (DFT) calculations are also employed to assess the magnitudes of the different interactions in the layer. Significantly, compared to other halogen bonds in physisorbed monolayers we have reported recently, the N⋯Br bond here is found to be non-linear. It is proposed that the increasing importance of the lateral hydrogen bond interactions, relative to the halogen bond strength, leads to the bending of the halogen bonds.

    DC Madden, I Temprano, M Sacchi, SJ Jenkins (2015)Spontaneous Local Symmetry Breaking: A Conformational Study of Glycine on Cu{311}, In: Journal of Physical Chemistry C119(23)pp. 13041-13049 American Chemical Society

    Understanding the interplay between intrinsic molecular chirality and chirality of the bonding footprint is crucial in exploiting enantioselectivity at surfaces. As such, achiral glycine and chiral alanine are the most obvious candidates if one is to study this interplay on different surfaces. Here, we have investigated the adsorption of glycine on Cu{311} using reflection–absorption infrared spectroscopy, low-energy electron diffraction, temperature-programmed desorption, and first-principles density-functional theory. This combination of techniques has allowed us to accurately identify the molecular conformations present under different conditions and discuss the overlayer structure in the context of the possible bonding footprints. We have observed coverage-dependent local symmetry breaking, with three-point bonded glycinate moieties forming an achiral arrangement at low coverages, and chirality developing with the presence of two-point bonded moieties at high coverages. Comparison with previous work on the self-assembly of simple amino acids on Cu{311} and the structurally similar Cu{110} surface has allowed us to rationalize the different conditions necessary for the formation of ordered chiral overlayers.

    H Hedgeland, Marco Sacchi, P Singh, AJ McIntosh, AP Jardine, G Alexandrowicz, DJ Ward, SJ Jenkins, W Allison, J Ellis (2016)Mass Transport in Surface Diffusion of van der Waals Bonded Systems - Boosted by Rotations?, In: Journal of Physical Chemistry Letters7(23)pp. 4819-4824 American Chemical Society

    Mass-transport at a surface is a key factor in heterogeneous catalysis. The rate is determined by excitation across a translational barrier and depends on the energy landscape and the coupling to the thermal bath of the surface. Here we use helium spin-echo spectroscopy (HeSE) to track the microscopic motion of benzene adsorbed on Cu(001) at low coverage (θ ∼ 0.07 ML). Specifically, our combined experimental and computational data determine both the absolute rate and mechanism of the molecular motion. The observed rate is significantly higher by a factor of 3.0±0.1 than is possible in a conventional, point-particle model and can only be understood by including additional molecular (rotational) coordinates. We argue that the effect can be described as an entropic contribution that enhances the population of molecules in the transition state. The process is generally relevant to molecular systems and illustrates the importance of the pre-exponential factor alongside the activation barrier in studies of surface kinetics.

    Richard Edward John Nicklin, Andrey Shavorskiy, Funda Aksoy Akgul, Zhi Liu, Roger Alexander Bennett, Marco Sacchi, Georg Held (2018)"Pop-On and Pop-Off" Surface Chemistry of Alanine on Ni{111} Under Elevated Hydrogen Pressures, In: The Journal of Physical Chemistry C122(14)pp. 7720-7730 American Chemical Society

    The co-adsorption of hydrogen with a simple chiral modifier, alanine, on Ni{111} was studied using Density Functional Theory in combination with ambient-pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy at temperatures of 300~K and above, which are representative of chiral hydrogenation reactions. Depending on the hydrogen pressure, the surface enables protons to "pop on and off" the modifier molecules, thus significantly altering the adsorption geometry and chemical nature of alanine from anionic tridentate in ultra-high vacuum to predominantly zwitterionic bidentate at hydrogen pressures above 0.1 Torr. This hydrogen-stabilised modifier geometry allows alternative mechanisms for proton transfer and the creation of enatioselective reaction environments.