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排序方式: 共有472条查询结果,搜索用时 15 毫秒
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Houk Jang Krishna P. Dhakal Kyung‐Il Joo Won Seok Yun Sachin M. Shinde Xiang Chen Soon Moon Jeong Suk Woo Lee Zonghoon Lee JaeDong Lee Jong‐Hyun Ahn Hyunmin Kim 《Advanced materials (Deerfield Beach, Fla.)》2018,30(14)
Understanding the collaborative behaviors of the excitons and phonons that result from light–matter interactions is important for interpreting and optimizing the underlying fundamental physics at work in devices made from atomically thin materials. In this study, the generation of exciton‐coupled phonon vibration from molybdenum disulfide (MoS2) nanosheets in a pre‐excitonic resonance condition is reported. A strong rise‐to‐decay profile for the transient second‐harmonic generation (TSHG) of the probe pulse is achieved by applying substantial (20%) beam polarization normal to the nanosheet plane, and tuning the wavelength of the pump beam to the absorption of the A‐exciton. The time‐dependent TSHG signals clearly exhibit acoustic phonon generation at vibration modes below 10 cm?1 (close to the Γ point) after the photoinduced energy is transferred from exciton to phonon in a nonradiative fashion. Interestingly, by observing the TSHG signal oscillation period from MoS2 samples of varying thicknesses, the speed of the supersonic waves generated in the out‐of‐plane direction (Mach 8.6) is generated. Additionally, TSHG microscopy reveals critical information about the phase and amplitude of the acoustic phonons from different edge chiralities (armchair and zigzag) of the MoS2 monolayers. This suggests that the technique could be used more broadly to study ultrafast physics and chemistry in low‐dimensional materials and their hybrids with ultrahigh fidelity. 相似文献
63.
Allemann C Gordillo R Clemente FR Cheong PH Houk KN 《Accounts of chemical research》2004,37(8):558-569
Computational studies have led to models to understand some classic and contemporary asymmetric reactions involving organocatalysts. The Hajos-Parrish-Eder-Sauer-Wiechert reaction and intermolecular aldol reactions as well as Mannich reactions and oxyaminations catalyzed by proline and other amino acids, and Diels-Alder reactions catalyzed by MacMillan's chiral amine organocatalysts have been studied with density functional theory. Quantitative predictions for several new catalysts and reactions are provided. 相似文献
64.
Renjith Devasia CP Reghunadhan Nair P Sivadasan KN Ninan 《Polymer International》2005,54(8):1110-1118
Polyacrylonitrile terpolymers of various compositions consisting of acrylonitrile (AN), itaconic acid (IA) and methyl acrylate (MA) were synthesized by solution polymerization in dimethylsulfoxide. Increase in concentration of either IA or MA retarded the overall polymerization rate and the polymer molecular weight. The system consisting of AN + MA and varying IA concentration was more prone to retardation in comparison with the system composed of AN + IA with variable MA concentration. The retardation factors were quantified. Minor quantities of MA boost the reactivity of IA in the terpolymer system. The terpolymer was richer in MA vis‐à‐vis the feed. The thermal characteristics of the terpolymer were examined as a function of its composition. In contrast to the copolymer of AN and IA requiring 1–1.5 mol% IA, the terpolymer required an IA content of approximately 2.5 mol% for optimum thermal stability. The polymer with 90 mol% AN, 2.5 mol% IA and 7.5 mol% MA exhibited reasonably good char‐forming characteristics and thermal stability. The overall crystallinity and crystallite size of the polymers were found to decrease on incorporation of the comonomers. The ‘aromatization index’ of the copolymer increased with the temperature of pyrolysis through re‐organization of the tetrahydropyridine ladder structure. Copyright © 2005 Society of Chemical Industry 相似文献
65.
MF McEntee KN Wright I Wanless R DeVovo JF Schneider R Shull 《Canadian Metallurgical Quarterly》1998,28(2):385-390
Seventy six consecutive patients with T2-4, N0-1, M0 primary breast cancer (BC) received a median of 3 cycles of CMF (cyclophosphamide, methotrexate, 5-fluorouracil) regimen. Tamoxifen was concomitantly administered in patients with estrogen receptor positive (ER+) BC. Ki67 antigen was evaluated immunohistochemically in tumor specimens obtained before chemotherapy and at mastectomy. At post chemotherapy evaluation, tumor shrinkage greater than 50% was obtained in 60 patients (78.9%), 21 of them being complete responders (27.6%). As a whole, primary chemotherapy significantly decreased the number of Ki67 positive cells. More than 50% decrease in Ki67 expression was observed in 78.9% of patients attaining a clinical complete response (CR), in 44.7% of patients with partial remission (PR) and in 50.0% of non-responders, while an increase (>25%) in Ki67 expression was found in 5.3%, 18.4% and 18.7% of patients with CR, PR and non-response, respectively. Both CR and PR rates were superimposable in patients with ER+ and ER- primary BC, while the reduction in Ki67 expression was mainly found in ER+ cases. Patients with increased Ki67 expression from baseline, at the end of primary chemotherapy, had a shorter disease-free interval (70 months) with respect to patients with no change (88+ months) or decrease (87+ months), p<0. 05. To conclude, the activity of CMF + tamoxifen in primary BC does not seem superior to that expected administering CMF alone. The reduction in Ki67 expression, as a whole, correlated with clinical CR, but some individual discrepancies between tumor shrinkage and Ki67 pattern have been observed. The Ki67 reduction mainly confined to the ER+ primary BC suggests that tumor response in this subset may be linked to the reduction in proliferation activity, whereas other mechanisms such as apoptosis might be responsible for the tumor shrinkage in ER- tumors. Since the increase in proliferation activity after primary chemotherapy was associated with a greater recurrence rate and lower disease free interval, irrespective of tumor response, changes in proliferation activity after primary chemotherapy may represent a potentially available parameter that, in addition to the tumor response, can discriminate patients who would benefit from the cytotoxic treatment from patients who would not. 相似文献
66.
EM Zelenkin KN Prozorovskaia AS Mirkin EG Zavgorodniaia AN Kandaurova 《Canadian Metallurgical Quarterly》1998,43(3):41-43
Effect of sodium hypochlorite disinfection of impressions on the size and quality of plaster models is studied. Twenty-minute submerging of silicone impressions in 0.5% sodium hypochlorite solution did not change their size and did not deteriorate the quality of surface and hardness of plaster models. Stomalgin impressions cannot be disinfected by sodium hypochlorite solution because of expressed destructive effect of this disinfectant on the impressions. 相似文献
67.
Twenty-seven patients who had a unilateral displaced intra-articular fracture of the calcaneus were managed with a cast instead of with reduction or an operation. The clinical result after a mean of six years (range, two to ten years) was excellent in five patients, good in five, fair in seven, and poor in ten. The sixteen patients who were re-examined for this study and for whom the gait was analyzed demonstrated abnormalities in ground-reaction force with regard to vertical force (F3) and temporal force factors (T2, T3, T6, and T9). Analysis of temporal and distance factors showed a trend toward a decreased proportion of single-limb support on the involved side. Three-dimensional motion analysis of the ankle and hindfoot was performed with electrogoniometers as the subject walked on a level surface, on a 10-degree side-slope, and up and down stairs. Motion was decreased in the sagittal, coronal, and transverse planes during walking on level ground and on a side-sloping surface. Significant decreases in motion in these planes were also seen during walking up and down stairs. Although these patients did not have a subsequent reconstructive operation, most had a residual functional deficit. 相似文献
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