Vibrational Sum Frequency Spectroscopic Investigation of the Azimuthal Anisotropy and Rotational Dynamics of Methyl-Terminated Silicon(111) Surfaces

Polarization-selected vibrational sum frequency generation spectroscopy (SFG) has been used to investigate the molecular orientation of methyl groups on CH_(3)-terminated Si(111) surfaces. The symmetric and asymmetric C–H stretch modes of the surface-bound methyl group were observed by SFG. Both methyl stretches showed a pronounced azimuthal anisotropy of the 3-fold symmetry in registry with the signal from the Si(111) substrate, indicating that the propeller-like rotation of the methyl groups was hindered at room temperature. The difference in the SFG line widths for the CH_3 asymmetric stretch that was observed for different polarization combinations (SPS and PPP for SFG, visible, and IR) indicated that the rotation proceeded on a 1–2 ps time scale, as compared to the 100 fs rotational dephasing of a free methyl rotor at room temperature

Combined Theoretical and Experimental Study of Band-Edge Control of Si through Surface Functionalization

The band-edge positions of H-, Cl-, Br-, methyl-, and ethyl-terminated Si(111) surfaces were investigated through a combination of density functional theory (DFT) and many-body perturbation theory, as well as by photoelectron spectroscopy and electrical device measurements. The calculated trends in surface potential shifts as a function of the adsorbate type and coverage are consistent with the calculated strength and direction of the dipole moment of the adsorbate radicals in conjunction with simple electronegativity-based expectations. The quasi-particle energies, such as the ionization potential (IP), that were calculated by use of many-body perturbation theory were in good agreement with experiment. The IP values that were calculated by DFT exhibited substantial errors, but nevertheless, the IP differences, i.e., IP_(R–Si(111))–IP_(H–Si(111)), computed using DFT were in good agreement with spectroscopic and electrical measurements

Hybridization of Surface Waves with Organic Adlayer Librations: A Helium Atom Scattering and Density Functional Perturbation Theory Study of Methyl-Si(111)

The interplay of the librations of a covalently bound organic adlayer with the lattice waves of an underlying semiconductor surface was characterized using helium atom scattering in conjunction with analysis by density functional perturbation teory. The Rayleigh wave dispersion relation of CH_(3)- and CD_(3)-terminated Si(111) surfaces was probed across the entire surface Brillouin zone by the use of inelastic helium atom time-of-flight experiments. The experimentally determined Rayleigh wave dispersion relations were in agreement with those predicted by density functional perturbation theory. The Rayleigh wave for the CH_(3)- and CD_(3)-terminated Si(111) surfaces exhibited a nonsinusoidal line shape, which can be attributed to the hybridization of overlayer librations with the vibrations of the underlying substrate. This combined synthetic, experimental, and theoretical effort clearly demonstrates the impact of hybridization between librations of the overlayer and the substrate lattice waves in determining the overall vibrational band structure of this complex interface

Electrical Junction Behavior of Poly(3,4-ethylenedioxythiophene) (PEDOT) Contacts to H‑Terminated and CH_3‑Terminated p‑, n‑, and n^+‑Si(111) Surfaces

The electronic and photovoltaic properties of junctions between the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and Si(111) surfaces have been investigated for a range of doping types, doping levels, and surface functionalization of the Si. PEDOT–poly(styrenesulfonate) (PSS) formed ohmic, low resistance contacts to H-terminated and CH_3-terminated p-type Si(111) surfaces. In contrast, PEDOT formed high barrier height (0.8–1.0 V) contacts to n-Si(111) surfaces, with CH_3-terminated n-Si(111)/PEDOT contacts showing slightly higher barrier heights (1.01 eV) than H-terminated n-Si(111)/PEDOT contacts (0.89 V). PEDOT contacts to CH_3-terminated and H-terminated n-Si(111) surfaces both produced photovoltages under illumination in accord with the Shockley diode limit based on bulk/recombination diffusion in the semiconductor. Such devices produced solar energy-conversion efficiencies of 5.7% under 100 mW cm^(–2) of simulated air mass 1.5 illumination. The electrical properties of PEDOT contacts to CH_3-terminated Si surfaces were significantly more stable in an air ambient than the electrical properties of PEDOT contacts to H-terminated Si surfaces. PEDOT films produced a low resistance, tunnel-barrier type of ohmic contact to n^+-Si(111) surfaces. Hence, through various combinations of doping type, doping level, and surface functionalization, the PEDOT/Si contact system offers a wide range of opportunities for integration into monolithic photovoltaic and/or artificial photosynthetic systems

Use of Mixed CH_3−/HC(O)CH_2CH_2−Si(111) Functionality to Control Interfacial Chemical and Electronic Properties During the Atomic-Layer Deposition of Ultrathin Oxides on Si(111)

Silicon surfaces terminated with a mixed monolayer containing both a propyl aldehyde functionality and methyl groups were prepared and used to control the interfacial chemical and electronic properties of Si(111) surfaces during atomiclayer deposition (ALD) of Al_2O_3 or MnO. Si(111) surfaces functionalized only with the aldehyde moiety exhibited surface recombination velocities, S, of 2500 ± 600 cm s^(−1) whereas the mixed CH_3−/HC(O)CH_2CH_2−Si(111) surfaces displayed S = 25 ± 7 cm s^(−1). During the ALD growth of either Al_2O_3 or MnO, both the HC(O)CH_2CH_2−Si(111) and CH_3−/HC(O)CH_2CH_2−Si(111) surfaces produced increased metal oxide deposition at low cycle number, relative to H−Si(111) or CH_3−Si(111) surfaces. As detected by X-ray photoelectron spectroscopy after the ALD process, the CH_3− and mixed CH_3−/HC(O)CH_2CH_2− functionalized Si(111) surfaces exhibited less interfacial SiO_x than was observed for ALD of metal oxides on H−Si(111) substrates

Comparison of the Photoelectrochemical Behavior of H‑Terminated and Methyl-Terminated Si(111) Surfaces in Contact with a Series of One-Electron, Outer-Sphere Redox Couples in CH_3CN

The photoelectrochemical behavior of methyl-terminated p-type and n-type Si(111) surfaces was determined in contact with a series of one-electron, outer-sphere, redox couples that span >1 V in the Nernstian redox potential, E(A/A^−), of the solution. The dependence of the current vs potential data, as well as of the open-circuit photovoltage, V_(OC), on E(A/A^−) was compared to the behavior of H-terminated p-type and n-type Si(111) surfaces in contact with these same electrolytes. For a particular E(A/A^−) value, CH_3-terminated p-Si(111) electrodes showed lower V_(OC) values than Hterminated p-Si(111) electrodes, whereas CH_3-terminated n-Si(111) electrodes showed higher V_(OC) values than H-terminated n-Si(111) electrodes. Under 100 mW cm^(−2) of ELH-simulated Air Mass 1.5 illumination, n-type H−Si(111) and CH_3−Si(111) electrodes both demonstrated nonrectifying behavior with no photovoltage at very negative values of E(A/A^−) and produced limiting V_(OC) values of >0.5 V at very positive values of E(A/A^−). Illuminated p-type H−Si(111) and CH_3−Si(111) electrodes produced no photovoltage at positive values of E(A/A^−) and produced limiting V_(OC) values in excess of 0.5 V at very negative values of E(A/A^−). In contact with CH_3CN-octamethylferrocene^(+/0), differential capacitance vs potential experiments yielded a −0.40 V shift in flat-band potential for CH_3-terminated n-Si(111) surfaces relative to H-terminated n-Si(111) surfaces. Similarly, in contact with CH_3CN-1,1′-dicarbomethoxycobaltocene^(+/0), the differential capacitance vs potential data indicated a −0.25 V shift in the flat-band potential for CH_3-terminated p-Si(111) electrodes relative to H-terminated p-Si(111) electrodes. The observed trends in V_(OC) vs E(A/A^−), and the trends in the differential capacitance vs potential data are consistent with a negative shift in the interfacial dipole as a result of methylation of the Si(111) surface. The negative dipole shift is consistent with a body of theoretical and experimental comparisons of the behavior of CH_3−Si(111) surfaces vs H−Si(111) surfaces, including density functional theory of the sign and magnitude of the surface dipole, photoemission spectroscopy in ultrahigh vacuum, the electrical behavior of Hg/Si contacts, and the pH dependence of the current−potential behavior of Si electrodes in contact with aqueous electrolytes

Timing of immune escape linked to success or failure of vaccination

Successful vaccination against HIV should limit viral replication sufficiently to prevent the emergence of viral immune escape mutations. Broadly directed immunity is likely to be required to limit opportunities for immune escape variants to flourish. We studied the emergence of an SIV Gag cytotoxic T cell immune escape variant in pigtail macaques expressing the Mane-A*10 MHC I allele using a quantitative RT-PCR to measure viral loads of escape and wild type variants. Animals receiving whole Gag expressing vaccines completely controlled an SIVmac251 challenge, had broader CTL responses and exhibited minimal CTL escape. In contrast, animals vaccinated with only a single CTL epitope and challenged with the same SIVmac251 stock had high levels of viral replication and rapid CTL escape. Unvaccinated na&iuml;ve animals exhibited a slower emergence of immune escape variants. Thus narrowly directed vaccination against a single epitope resulted in rapid immune escape and viral levels equivalent to that of na&iuml;ve unvaccinated animals. These results emphasize the importance of inducing broadly directed HIV-specific immunity that effectively quashes early viral replication and limits the generation of immune escape variants. This has important implications for the selection of HIV vaccines for expanded human trials.<br /

Determinants of Depressive Symptoms at 1 Year Following ICU Discharge in Survivors of $ 7 Days of Mechanical Ventilation : Results From the RECOVER Program, a Secondary Analysis of a Prospective Multicenter Cohort Study

Abstract : Background: Moderate to severe depressive symptoms occur in up to one-third of patients at 1 year following ICU discharge, negatively affecting patient outcomes. This study evaluated patient and caregiver factors associated with the development of these symptoms. Methods: This study used the Rehabilitation and Recovery in Patients after Critical Illness and Their Family Caregivers (RECOVER) Program (Phase 1) cohort of 391 patients from 10 medical/surgical university-affiliated ICUs across Canada. We determined the association between patient depressive symptoms (captured by using the Beck Depression Inventory II [BDI-II]), patient characteristics (age, sex, socioeconomic status, Charlson score, and ICU length of stay [LOS]), functional independence measure (FIM) motor subscale score, and caregiver characteristics (Caregiver Assistance Scale and Center for Epidemiologic Studies-Depression Scale) by using linear mixed models at time points 3, 6, and 12 months. Results: BDI-II data were available for 246 patients. Median age at ICU admission was 56 years (interquartile range, 45-65 years), 143 (58%) were male, and median ICU LOS was 19 days (interquartile range, 13-32 days). During the 12-month follow-up, 67 of 246 (27.2%) patients had a BDI-II score ≥ 20, indicating moderate to severe depressive symptoms. Mixed models showed worse depressive symptoms in patients with lower FIM motor subscale scores (1.1 BDI-II points per 10 FIM points), lower income status (by 3.7 BDI-II points; P = .007), and incomplete secondary education (by 3.8 BDI-II points; P = .009); a curvilinear relation with age (P = .001) was also reported, with highest BDI-II at ages 45 to 50 years. No associations were found between patient BDI-II and comorbidities (P = .92), sex (P = .25), ICU LOS (P = .51), or caregiver variables (Caregiver Assistance Scale [P = .28] and Center for Epidemiologic Studies Depression Scale [P = .74]). Conclusions: Increased functional dependence, lower income, and lower education are associated with increased severity of post-ICU depressive symptoms, whereas age has a curvilinear relation with symptom severity. Knowledge of risk factors may inform surveillance and targeted mental health follow-up. Early mobilization and rehabilitation aiming to improve function may serve to modify mood disorders

Thriving at work: A meta‐analysis

Thriving at work refers to a positive psychological state characterized by a joint senseof vitality and learning. On the basis of Spreitzer and colleagues' model, we present acomprehensive meta‐analysis of antecedents and outcomes of thriving at work(K= 73 independent samples,N= 21,739 employees). Results showed that thrivingat work is associated with individual characteristics, such as psychological capital(rc= .47), proactive personality (rc= .58), positive affect (rc= .52), and work engage-ment (rc= .64). Positive associations were also found between thriving at work andrelational characteristics, including supportive coworker behavior (rc= .42), support-ive leadership behavior (rc= .44), and perceived organizational support (rc= .63).Moreover, thriving at work is related to important employee outcomes, includinghealth‐related outcomes such as burnout (rc=−.53), attitudinal outcomes such ascommitment (rc= .65), and performance‐related outcomes such as task performance(rc= .35). The results of relative weights analyses suggest that thriving exhibits small,albeit incremental predictive validity above and beyond positive affect and workengagement, for task performance, job satisfaction, subjective health, and burnout.Overall, the findings of this meta‐analysis support Spreitzer and colleagues' modeland underscore the importance of thriving in the work conte