Should surface science exploit more quantitative experiments?

In recent years two particular methods, scanning probe microscopy and theoretical total energy calculations (based, particularly, on density functional theory), have led to major advances in our understanding of surface science. However, performed to the exclusion of more ‘traditional’ experimental methods that provide quantitative information on the composition, vibrational properties, adsorption and desorption energies, and on the electronic and geometrical structure, the interpretation of the results can be unnecessarily speculative. Combined with these methods, on the other hand, they give considerable added power to the long-learnt lesson of the need to use a range of complementary techniques to unravel the complexities of surface phenomena

The role of reconstruction in self-assembly of alkylthiolate monolayers on coinage metal surfaces

Through a combination of standard laboratory-based surface science methods, together with synchrotron radiation-based normal incidence X-ray standing wave (NIXSW) experiments, the interface structure of simple alkylthiolate ‘self-assembled monolayers’ on Cu(1 1 1), Ag(1 1 1) and Au(1 1 1) has been investigated over the last not, vert, similar15 years. A key conclusion is that in all cases the adsorbate produces a substantial, density-lowering, reconstruction of the outermost metal layer, although the nature of these reconstructions is quite different on the three metals. The main results of these investigations are briefly reviewed and contrasted

Time lower bounds for nonadaptive turnstile streaming algorithms

We say a turnstile streaming algorithm is "non-adaptive" if, during updates, the memory cells written and read depend only on the index being updated and random coins tossed at the beginning of the stream (and not on the memory contents of the algorithm). Memory cells read during queries may be decided upon adaptively. All known turnstile streaming algorithms in the literature are non-adaptive. We prove the first non-trivial update time lower bounds for both randomized and deterministic turnstile streaming algorithms, which hold when the algorithms are non-adaptive. While there has been abundant success in proving space lower bounds, there have been no non-trivial update time lower bounds in the turnstile model. Our lower bounds hold against classically studied problems such as heavy hitters, point query, entropy estimation, and moment estimation. In some cases of deterministic algorithms, our lower bounds nearly match known upper bounds

Water does partially dissociate on the perfect TiO2(110) surface : a quantitative structure determination

There has been a long-standing controversy as to whether water can dissociate on perfect areas of a TiO2(110) surface; most early theoretical work indicated this dissociation was facile, while experiments indicated little or no dissociation. More recently the consensus of most theoretical calculations is that no dissociation occurs. New results presented here, based on analysis of scanned-energy mode photoelectron diffraction data from the OH component of O 1s photoemission, show the coexistence of molecular water and OH species in both atop (OHt) and bridging (OHbr) sites. OHbr can arise from reaction with oxygen vacancy defect sites (Ovac), but OHt have only been predicted to arise from dissociation on the perfect areas of the surface. The relative concentrations of OHt and OHbr sites arising from these two dissociation mechanisms are found to be fully consistent with the initial concentration Ovac sites, while the associated Ti-O bondlengths of the OHt and OHbr species are found to be 1.85±0.08Å and 1.94±0.07 Å, respectively

When is capital enough to get female microenterprises growing? Evidence from a randomized experiment in Ghana

Standard models of investment predict that credit-constrained firms should grow rapidly when given additional capital, and that how this capital is provided should not effect decisions to invest in the business or consume the capital. We randomly gave cash and in-kind grants to male- and female-owned microenterprises in urban Ghana. Our findings cast doubt on the ability of capital alone to stimulate the growth of female microenterprises. First, while the average treatment effects of the in-kind grants are large and positive for both males and females, the gain in profits is almost zero for women with initial profits below the median, suggesting that capital alone is not enough to grow subsistence enterprises owned by women. Second, for women we strongly reject equality of the cash and in-kind grants; only in-kind grants lead to growth in business profits. The results for men also suggest a lower impact of cash, but differences between cash and in-kind grants are less robust. The difference in the effects of cash and in-kind grants is associated more with a lack of self-control than with external pressure. As a result, the manner in which funding is provided affects microenterprise growth

Bridging the pressure gap : can we get local quantitative structural information at ‘near-ambient’ pressures?

In recent years there have been an increasing number of investigations aimed at ‘bridging the pressure gap’ between UHV surface science experiments on well-characterised single crystal surfaces and the much higher (ambient and above) pressures relevant to practical catalyst applications. By applying existing photon-in/photon-out methods and developing instrumentation to allow photoelectron emission to be measured in higher-pressure sample environments, it has proved possible to obtain surface compositions and spectroscopic fingerprinting of chemical and molecular states of adsorbed species at pressures up to a few millibars. None of these methods, however, provide quantitative structural information on the local adsorption sites of isolated atomic and molecular adsorbate species under these higher-pressure reaction conditions. Methods for gaining this information are reviewed and evaluated

How does your crystal grow? : A commentary on Burton, Cabrera and Frank (1951) 'The growth of crystals and the equilibrium structure of their surfaces'

The key ideas presented in the classic paper ‘The growth of crystals and the equilibrium structure of their surfaces’ by W. K. Burton, N. Cabrera and F. C. Frank, published in Philosophical Transactions A in 1951, are summarized and put in the context of both the state of knowledge at the time of publication and the considerable amount of work since that time that has built on and developed these ideas. Many of these developments exploit the huge increase in the capabilities of computer modelling that complement the original analytic approach of the paper. The dearth of relevant experimental data at the time of the original publication has been transformed by the application of increasingly sophisticated modern methods of surface science. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society