A new approach to materials discovery for electronic and thermoelectric properties of single-molecule junctions

We have investigated a large set of symmetric and asymmetric molecules to demonstrate a general rule for molecular-scale quantum transport, which provides a new route to materials design and discovery. The rule states “the conductance GXBY of an asymmetric molecule is the geometric mean of the conductance of the two symmetric molecules derived from it and the thermopower SXBY of the asymmetric molecule is the algebraic mean of their thermopowers”. The studied molecules have a structure X-B-Y, where B is the backbone of the molecule, while X and Y are anchor groups, which bind the molecule to metallic electrodes. When applied to experimentally-measured histograms of conductance and thermopower, the rules apply to the statistically-most-probable values. We investigated molecules with anchors chosen from the following family: cyano, pyridl, dihydrobenzothiol, amine and thiol. For the backbones B, we tested fourteen different structures. We found that the formulae (GXBY)2 = GXBX*GYBY and SXBY=(SXBX+SYBY)/2 were satisfied in the large majority of the cases, provided the Fermi energy is located within the HOMO-LUMO gap of the molecules. The circuit rules imply that if measurements are performed on molecules with nA different anchors and nB different backbones, then properties of nA(nA+1)nB/2 molecules can be predicted. So for example, in the case of 20 backbones and 10 anchors, 30 measurements (or reliable calculations) can provide a near quantitative estimate for 1070 measurements of other molecules, no extra cost

Identification of a positive-Seebeck-coefficient exohedral fullerene

If fullerene-based thermoelectricity is to become a viable technology, then fullerenes exhibiting both positive and negative Seebeck coefficients are needed. C60 is known to have a negative Seebeck coefficient and therefore in this paper we address the challenge of identifying a positive-Seebeck-coefficient fullerene. We investigated the thermoelectric properties of single-molecule junctions of the exohedral fullerene C50Cl10 connected to gold electrodes and found that it indeed possesses a positive Seebeck coefficient. Furthermore, in common with C60, the Seebeck coefficient can be increased by placing more than one C50Cl10 in series. For a single C50Cl10, we find S = +8 μV K−1 and for two C50Cl10's in series we find S = +30 μV K−1. We also find that the C50Cl10 monomer and dimer have power factors of 0.5 × 10−5 W m−1 K−2 and 6.0 × 10−5 W m−1 K−2 respectively. These results demonstrate that exohedral fullerenes provide a new class of thermoelectric materials with desirable properties, which complement those of all-carbon fullerenes, thereby enabling the boosting of the thermovoltage in all-fullerene tandem structures

Tuning the thermoelectric properties of metallo-porphyrins

We investigated the thermoelectric properties of metalloporphyrins connected by thiol anchor groups to gold electrodes. By varying the transition metal-centre over the family Mn, Co, Ni, Cu, Fe, and Zn we are able to tune the molecular energy levels relative to the Fermi energy of the electrodes. The resulting single-molecule room-temperature thermopowers range from almost zero for Co and Cu centres, to +80 μV K−1 and +230 μV K−1 for Ni and Zn respectively. In contrast, the thermopowers with Mn(II) or Fe(II) metal centres are negative and lie in the range −280 to −260 μV K−1. Complexing these with a counter anion to form Fe(III) and Mn(III) changes both the sign and magnitude of their thermopowers to +218 and +95 respectively. The room-temperature power factors of Mn(II), Mn(III), Fe(III), Zn and Fe(II) porphyrins are predicted to be 5.9 × 10−5 W m−1 K−2, 5.4 × 10−4 W m−1 K−2, 9.5 × 10−4 W m−1 K−2, 1.6 × 10−4 W m−1 K−2 and 2.3 × 10−4 W m−1 K−2 respectively, which makes these attractive materials for molecular-scale thermoelectric devices

Quasiparticle and excitonic gaps of one-dimensional carbon chains

We report diffusion quantum Monte Carlo (DMC) calculations of the quasiparticle and excitonic gaps of hydrogen-terminated oligoynes and polyyne. The electronic gaps are found to be very sensitive to the atomic structure in these systems. We have therefore optimised the geometry of polyyne by directly minimising the DMC energy with respect to the lattice constant and the Peierls-induced carbon-carbon bond-length alternation. We find the bond-length alternation of polyyne to be 0.136(2) Å and the excitonic and quasiparticle gaps to be 3.30(7) and 3.4(1) eV, respectively. The DMC zone-centre longitudinal optical phonon frequency of polyyne is 2084(5) cm-1, which is consistent with Raman spectroscopic measurements for large oligoynes

Adding web-based support to exercise referral schemes improves symptoms of depression in people with elevated depressive symptoms:A secondary analysis of the e-coachER randomised controlled trial

Background: Exercise referral schemes (ERS) reduce depression but the additional effect on mental health from web-based behavioural support is unknown. The e-coachER trial reported no effect of augmenting usual ERS with theory-driven web-based behavioural support on moderate to vigorous physical activity (MVPA) at 12 months for patients with chronic physical and mental health conditions. The present study reports the effects of the e-coachER intervention on depression, anxiety and MVPA only among participants with elevated depressive symptoms and investigates whether these were mediated by changes in MVPA and hypothesised cognitive and behavioural processes. Methods: Of the original 450 adults recruited into the e-coachER trial, 205 had at least mild depression, based on the Hospital Anxiety and Depression Scale (HADS), and were included in the present analysis. Data collected included the HADS, accelerometer measured and self-reported MVPA and survey process measures on physical activity action planning, self-monitoring and goal reviewing, and perceived importance, confidence, competence, autonomy and support. Linear mixed models were used to compare groups for change in depression and anxiety at 4 and 12 months using intention-to-treat complete case analysis, controlling for baseline. We also examined whether changes in physical activity and process variables at 4 months mediated changes in depression and anxiety at 12 months. Results: Of the 205 participants, 138 (67%) provided follow-up data at four months and 126 (61%) at 12 months. For those that provided follow-up data, those randomised to e-coachER reported improved levels of depression (−1.36, 95% CI: −2.55 to −0.18) but not anxiety, or MVPA, compared with controls at four months. No differences were observed at 12 months for depression, anxiety or MVPA. Intervention effects on accelerometer-measured or self-reported MVPA did not mediate improvements in depression or anxiety. However, intervention effects on confidence, competence and self-monitoring at four months significantly mediated the reduction in depression scores at four months. Intervention effects on competence and self-monitoring at four months also significantly mediated improvements in anxiety scores at four months. Interpretation: Adding web-based support to usual ERS leads to reductions in depression but not anxiety at four months. Changes in depression and anxiety were influenced by changing people's motivational regulations toward physical activity. The benefit of adding web-based support to usual ERS on mental health appears to be from increasing a sense of confidence, competence and self-monitoring rather than from increasing physical activity in people with elevated depression. ERS should focus more on strengthening motivational regulations than just doing more exercise. Trial registration: ISRCTN15644451.</p

Adding web-based support to exercise referral schemes improves symptoms of depression in people with elevated depressive symptoms:A secondary analysis of the e-coachER randomised controlled trial

Background: Exercise referral schemes (ERS) reduce depression but the additional effect on mental health from web-based behavioural support is unknown. The e-coachER trial reported no effect of augmenting usual ERS with theory-driven web-based behavioural support on moderate to vigorous physical activity (MVPA) at 12 months for patients with chronic physical and mental health conditions. The present study reports the effects of the e-coachER intervention on depression, anxiety and MVPA only among participants with elevated depressive symptoms and investigates whether these were mediated by changes in MVPA and hypothesised cognitive and behavioural processes. Methods: Of the original 450 adults recruited into the e-coachER trial, 205 had at least mild depression, based on the Hospital Anxiety and Depression Scale (HADS), and were included in the present analysis. Data collected included the HADS, accelerometer measured and self-reported MVPA and survey process measures on physical activity action planning, self-monitoring and goal reviewing, and perceived importance, confidence, competence, autonomy and support. Linear mixed models were used to compare groups for change in depression and anxiety at 4 and 12 months using intention-to-treat complete case analysis, controlling for baseline. We also examined whether changes in physical activity and process variables at 4 months mediated changes in depression and anxiety at 12 months. Results: Of the 205 participants, 138 (67%) provided follow-up data at four months and 126 (61%) at 12 months. For those that provided follow-up data, those randomised to e-coachER reported improved levels of depression (−1.36, 95% CI: −2.55 to −0.18) but not anxiety, or MVPA, compared with controls at four months. No differences were observed at 12 months for depression, anxiety or MVPA. Intervention effects on accelerometer-measured or self-reported MVPA did not mediate improvements in depression or anxiety. However, intervention effects on confidence, competence and self-monitoring at four months significantly mediated the reduction in depression scores at four months. Intervention effects on competence and self-monitoring at four months also significantly mediated improvements in anxiety scores at four months. Interpretation: Adding web-based support to usual ERS leads to reductions in depression but not anxiety at four months. Changes in depression and anxiety were influenced by changing people's motivational regulations toward physical activity. The benefit of adding web-based support to usual ERS on mental health appears to be from increasing a sense of confidence, competence and self-monitoring rather than from increasing physical activity in people with elevated depression. ERS should focus more on strengthening motivational regulations than just doing more exercise. Trial registration: ISRCTN15644451.</p

A process evaluation, with mediation analysis, of a web-based intervention to augment primary care exercise referral schemes:the e-coachER randomised controlled trial

BACKGROUND: The e-coachER trial aimed to determine whether adding web-based behavioural support to exercise referral schemes (ERS) increased long-term device-measured physical activity (PA) for patients with chronic conditions, compared to ERS alone, within a randomised controlled trial. This study explores the mechanisms of action of the e-coachER intervention using measures of the behaviour change processes integral to the intervention's logic model.METHODS: Four hundred fifty adults with obesity, diabetes, hypertension, osteoarthritis or history of depression referred to an ERS were recruited in Plymouth, Birmingham and Glasgow. The e-coachER intervention comprising 7-Steps to Health was aligned with Self-Determination Theory and mapped against evidence-based behaviour change techniques (BCTs). Participants completed questionnaires at 0, 4, and 12 months to assess PA and self-reported offline engagement with core BCTs in day-to-day life (including action planning and self-monitoring) and beliefs relating to PA (including perceived importance, confidence, competence, autonomy and support). We compared groups at 4 and 12 months, controlling for baseline measures and other covariates. Mediation analysis using the product of coefficients method was used to determine if changes in process variables mediated intervention effects on moderate to vigorous physical activity (MVPA) recorded by accelerometer and self-report at 4- and 12-months.RESULTS: The internal reliability (Cronbach's alpha) for all multi-item scales was &gt; 0.77. At 4-months, those randomised to e-coachER reported higher levels of PA beliefs relating to importance (1.01, 95% confidence interval (CI): 0.42 to 1.61, p = 0.001), confidence (1.28, 95% CI: 0.57 to 1.98, p &lt; 0.001), competence (1.61, 95% CI: .68 to 2.54, p = 0.001), availability of support (0.77, 95% CI: 0.07 to 1.48, p = 0.031), use of action planning (1.54, 95% CI: 0.23 to 2.85, p = 0.021) and use of self-monitoring (0.76, 95% CI: 0.19 to 1.32, p = 0.009) compared to ERS alone. There were no intervention effects on autonomous beliefs or perceived frequency of support, compared to ERS alone. At the 12-month follow-up, participants belief in the importance of PA was the only process measure to remain significantly higher in the e-coachER group when compared to ERS alone (0.75, 95% CI: 0.05 to 1.45). Intervention effects on perceived importance (2.52, 95% CI: 0.45 to 5.39), action planning (1.56, 95% CI: 0.10 to 3.54) and self-monitoring (1.92, 95% CI: 0.21 to 4.33) at 4-months significantly mediated change in accelerometer measured MVPA at 12-months (recorded in ≥ 10-min bouts).CONCLUSIONS: e-coachER led to some short-term changes in most process outcomes. Some of these processes also appeared to mediate e-coachER effects on changes in accelerometer measured MVPA. Further work should be carried out to understand how best to design and implement theoretically underpinned web-based physical activity promotion interventions within ERS.TRIAL REGISTRATION: ISRCTN, ISRCTN15644451 . Registered 12 February 2015.</p

A C60-aryne building block:synthesis of a hybrid all-carbon nanostructure

The synthesis of a [60]fullerene-benzyne building block and its further chemical cycloaddition reaction with graphene has resulted in a new all-carbon hybrid material which has been characterized by TGA, FTIR and Raman spectroscopies, XPS as well as TEM. Based on computational studies,the formation of both [2+2] and [4+2] cycloadducts on the graphene surface is feasible

Charge transport through dicarboxylic-acid-terminated alkanes bound to graphene-gold nanogap electrodes

Graphene-based electrodes are attractive for single-molecule electronics due to their high stability and conductivity and reduced screening compared with metals. In this paper, we use the STM-based matrix isolation I(s) method to measure the performance of graphene in single-molecule junctions with one graphene electrode and one gold electrode. By measuring the length dependence of the electrical conductance of dicarboxylic-acid-terminated alkanes, we find that transport is consistent with phase-coherent tunneling, but with an attenuations factor βN = 0.69 per methyl unit, which is lower than the value measured for Au-molecule-Au junctions. Comparison with density-functional-theory calculations of electron transport through graphene-molecule-Au junctions and Au-molecule-Au junctions reveals that this difference is due to the difference in Fermi energies of the two types of junction, relative to the frontier orbitals of the molecules. For most molecules, their electrical conductance in graphene-molecule-Au junctions is higher than that in Au-molecule-Au junctions, which suggests that graphene offers superior electrode performance, when utilizing carboxylic acid anchor groups