Daniel M. Packwood

Chemical and entropic control on the molecular self-assembly process

Molecular self-assembly refers to the spontaneous assembly of molecules into larger structures. In order to exploit molecular self-assembly for the bottom-up synthesis of nanomaterials, the effects of chemical control (strength of the directionality in the intermolecular interaction) and entropic control (temperature) on the self-assembly process should be clarified. Here we present a theoretical methodology that unambiguously distinguishes the effects of chemical and entropic control on the self-assembly of molecules adsorbed to metal surfaces. While chemical control simply increases the formation probability of ordered structures, entropic control induces a variety of effects. These effects range from fine structure modulation of ordered structures, through to degrading large, amorphous structures into short, chain-shaped structures. Counterintuitively, the latter effect shows that entropic control can improve molecular ordering. By identifying appropriate levels of chemical and entropic control, our methodology can, therefore, identify strategies for optimizing the yield of desired nanostructures from the molecular self-assembly process.

A Hydroxamic Acid Anchoring Group for Durable Dye-Sensitized Solar Cells Incorporating a Cobalt Redox Shuttle

A hydroxamic acid group has been employed for the first time as an anchoring group for cobalt-based dye-sensitized solar cells (DSSCs). The porphyrin dye YD2-o-C8HA including a hydroxamic acid anchoring group exhibited a power conversion efficiency (η) of 6.4 %, which is close to that of YD2-o-C8, a representative porphyrin dye incorporating a conventional carboxylic acid. More importantly, YD2-o-C8HA was found to be superior to YD2-o-C8 in terms of both binding ability to TiO2 and durability of cobalt-based DSSCs. Notably, YD2-o-C8HA photocells revealed a higher η-value (4.1 %) than YD2-o-C8 (2.8 %) after 500 h illumination. These results suggest that the hydroxamic acid can be used for DSSCs with other transition-metal-based redox shuttle to ensure high cell durability as well as excellent photovoltaic performance.

pH-Dependent Wettability of Carboxyphenyl Films Grafted to Glassy Carbon

Surfaces than can switch their properties in response to external stimuli are of fundamental as well as technological interest. A prerequisite for successful switching in thin surface layers is sufficient free volume in the layer to allow molecular motions or reactions. Multilayer films grafted from aryldiazonium salts have a loosely packed structure and are good candidates for preparation of switchable surfaces. In this work, the pH-dependent wettability of carboxyphenyl films on glassy carbon surfaces is examined using water contact angle measurements. The film structure is manipulated by exposing freshly grafted films to solvents of different polarity; this influences the wettability differences observed between low- and high-pH measurements. The order of measurement of contact angles (from low pH to high, or vice versa) also influences the pH-dependent wettability. The results are consistent with film reorganization, including the formation of dimeric hydrogen-bonded structures, in response to the polarity and pH of the surrounding medium.

Non-equilibrium thermodynamics of the gas–liquid interface: Measurement of the Onsager heat of transport for carbon dioxide at the surface of water

Abstract The Onsager heat of transport, Q*, for nitrous oxide at the surface of water has been measured by using high-resolution diode-laser spectroscopy to monitor the partial pressure of N2O as a function of the temperature difference across a 5-mm vapour gap above the water surface. The metal surface above the vapour gap was conditioned with ammonia prior to the measurements, to enable efficient thermal accommodation at that surface, which allowed the temperature difference to be as large as 15 K. We find Q* = –6.6 ± 0.85 kJ mol–1, so that Q*/RT is –2.9 at T = 275.15 K. These experiments are a necessary preliminary to a planned series of measurements of Q* for CO2 at the surface of simulated sea water.

Construction of a Hierarchical Architecture of Covalent Organic Frameworks via a Postsynthetic Approach

Covalent organic frameworks (COFs) represent an emerging class of crystalline porous materials that are constructed by the assembly of organic building blocks linked via covalent bonds. Several strategies have been developed for the construction of new COF structures; however, a facile approach to fabricate hierarchical COF architectures with controlled domain structures remains a significant challenge, and has not yet been achieved. In this study, a dynamic covalent chemistry (DCC)-based postsynthetic approach was employed at the solid-liquid interface to construct such structures. Two-dimensional imine-bonded COFs having different aromatic groups were prepared, and a homogeneously mixed-linker structure and a heterogeneously core-shell hollow structure were fabricated by controlling the reactivity of the postsynthetic reactions. Solid-state nuclear magnetic resonance (NMR) spectroscopy and transmission electron microscopy (TEM) confirmed the structures. COFs prepared by a postsynthetic approach exhibit several functional advantages compared with their parent phases. Their Brunauer-Emmett-Teller (BET) surface areas are 2-fold greater than those of their parent phases because of the higher crystallinity. In addition, the hydrophilicity of the material and the stepwise adsorption isotherms of H2O vapor in the hierarchical frameworks were precisely controlled, which was feasible because of the distribution of various domains of the two COFs by controlling the postsynthetic reaction. The approach opens new routes for constructing COF architectures with functionalities that are not possible in a single phase.

Two-dimensional molecular magnets with weak topological invariant magnetic moments: mathematical prediction of targets for chemical synthesis.

An open problem in applied mathematics is to predict interesting molecules that are realistic targets for chemical synthesis. In this paper, we use a spin Hamiltonian-type model to predict molecular magnets (MMs) with magnetic moments that are intrinsically robust under random shape deformations to the molecule. Using the concept of convergence in probability, we show that for MMs in which all spin centres lie in-plane and all spin centre interactions are ferromagnetic, the total spin of the molecule is a ‘weak topological invariant’ when the number of spin centres is sufficiently large. By weak topological invariant, we mean that the total spin of the molecule depends only upon the arrangement of spin centres in the molecule, and is unlikely to change under shape deformations to the molecule. Our calculations show that only between 20 and 50 spin centres are necessary for the total spin of these MMs to be a weak topological invariant. The robustness effect is particularly enhanced for two-dimensional ferromagnetic MMs that possess a small number of spin rings in the structure.

Dephasing by a continuous-time random walk process.

Stochastic treatments of magnetic resonance spectroscopy and optical spectroscopy require evaluations of functions such as (exp(i ∫(0)(t) Q(s)ds)), where t is time, Q(s) is the value of a stochastic process at time s, and the angular brackets denote ensemble averaging. This paper gives an exact evaluation of these functions for the case where Q is a continuous-time random walk process. The continuous-time random walk describes an environment that undergoes slow steplike changes in time. It also has a well-defined Gaussian limit and so allows for non-Gaussian and Gaussian stochastic dynamics to be studied within a single framework. We apply the results to extract qubit-lattice interaction parameters from dephasing data of P-doped Si semiconductors (data collected elsewhere) and to calculate the two-dimensional spectrum of a three-level harmonic oscillator undergoing random frequency modulations.

A Stochastic, Local Mode Treatment of High-Energy Gas—Liquid Collisions

The scattering angle distributions of high-energy molecular beams at the surfaces of three different liquids are treated in terms local mode theory. This is achieved by setting up a stochastic process modeling the effect of a superposition of local mode surface displacements on the incoming particles trajectory. The results are found to be in good qualitative agreement with experiment, and directions for further work are indicated.

Atomic collision effects during PLD processes: nonstoichiometry control in transparent superconductors

The pulsed laser deposition (PLD) growth processes of spinel lithium titanates, Li4Ti5O12 and LiTi2O4, on MgAl2O4 (111) substrates are investigated. Although a Li4Ti5O12 target was used for the depositions, the Li/Ti atomic ratio of the species arriving at the substrate during deposition was only ~0.5, enabling high quality LiTi2O4 films to be prepared with a rocking curve full-width at half-maximum of ~0.05°. The LiTi2O4 epitaxial thin films exhibited high conductivity at room temperature (~3.0 × 103 Ω−1cm−1) and a superconducting transition temperature of ~13.3 K. These values are the highest recorded for epitaxial thin films. Moreover, the effect of collisions between the atoms in a plume were studied quantitatively. These results demonstrate the importance of the target composition, providing further insight into Licontaining metal-oxide deposition processes using PLD.

Overview of Bayesian Optimization in Materials Science

Like any other field of research, materials science involves a lot of trial and error: in the process of creating a new material or device, we will inevitably make several prototypes which fail to perform as hoped.