全文获取类型
收费全文 | 12277篇 |
免费 | 3312篇 |
国内免费 | 1391篇 |
专业分类
化学 | 4625篇 |
晶体学 | 85篇 |
力学 | 41篇 |
综合类 | 77篇 |
数学 | 813篇 |
物理学 | 11339篇 |
出版年
2024年 | 61篇 |
2023年 | 224篇 |
2022年 | 634篇 |
2021年 | 598篇 |
2020年 | 567篇 |
2019年 | 475篇 |
2018年 | 427篇 |
2017年 | 438篇 |
2016年 | 616篇 |
2015年 | 567篇 |
2014年 | 738篇 |
2013年 | 984篇 |
2012年 | 776篇 |
2011年 | 910篇 |
2010年 | 710篇 |
2009年 | 738篇 |
2008年 | 931篇 |
2007年 | 906篇 |
2006年 | 894篇 |
2005年 | 757篇 |
2004年 | 657篇 |
2003年 | 560篇 |
2002年 | 518篇 |
2001年 | 377篇 |
2000年 | 440篇 |
1999年 | 275篇 |
1998年 | 297篇 |
1997年 | 189篇 |
1996年 | 114篇 |
1995年 | 104篇 |
1994年 | 70篇 |
1993年 | 61篇 |
1992年 | 60篇 |
1991年 | 47篇 |
1990年 | 51篇 |
1989年 | 47篇 |
1988年 | 24篇 |
1987年 | 17篇 |
1986年 | 26篇 |
1985年 | 21篇 |
1984年 | 18篇 |
1983年 | 7篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1979年 | 3篇 |
1978年 | 7篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1972年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
A new and convenient route is developed to synthesize CdSe and core-shell CdSe/CdS quantum dots(QDs) in aqueous solution.The gaseous precursors,H2Se and H2S,generated on-line by reducing SeO 3 2à with NaBH 4 and the reaction between Na 2 S and diluted H2SO 4,are used to form high-quality CdSe and CdSe/CdS QDs,respectively.The synthesized water-soluble CdSe and CdSe/CdS QDs possess high quantum yield(3% and 20%) and narrow full-width-at-half-maximum(43 nm and 38 nm).The synthesis process is easily reproducible with simple apparatus and low-toxic chemicals,and can be readily extended to the large-scale aqueous synthesis of QDs. 相似文献
992.
利用电化学方法在碱性条件下电解石墨棒, 通过常温下水合肼还原, 得到5~10 nm的荧光石墨烯量子点(Graphene Quantum Dots, GQDs). 通过透射电子显微镜(TEM)、原子力显微镜(AFM)对所制备的GQDs进行形貌表征, GQDs的粒子大小均一, 为单层石墨烯. 通过傅里叶变换红外光谱(FTIR)、荧光光谱(PL)、紫外可见吸收光谱(UV-vis)、X 射线衍射光谱(XRD)对所制备的GQDs进行性质测定, 发现GQDs可以发出黄色荧光, 量子产率为14%, 毒性低、具有良好的水溶性、荧光稳定性和生物兼容性, 可顺利进入细胞, 在肿瘤细胞的成像研究方面具有广泛的应用前景. 相似文献
993.
荧光量子点及其在生物检测中的应用 总被引:1,自引:0,他引:1
量子点(QDs)是一种零维的半导体纳米晶体,与传统的有机染料相比,具有独特的光学特征。由于它们具有激发光谱宽、发射光谱窄、发射波长精确可调、量子产率高和荧光稳定性好等特点,作为新一代的生物荧光探针,已被广泛应用于生物检测。本文介绍了QDs的基本概念和性质,探讨了QDs的制备方法及表面修饰,对其毒性也作了简要分析,提供了QDs在荧光免疫分析、生物芯片、生物传感器及体内成像等方面的应用实例。随着技术发展的不断进展,QDs在生物分析领域有着更为广泛的潜在的应用前景。 相似文献
994.
995.
996.
Kovácik V Bekesová S Tvaroska I Hirsch J Chmelík J 《Journal of mass spectrometry : JMS》2004,39(12):1554-1561
Fragmentation mechanisms of electrospray ionization (ESI) mass spectrometry of aldobiuronic and pseudoaldobiuronic acid derivatives were elucidated by multistage mass spectrometric (MS(n), n = 2-5) measurements of selected ions. Characteristic under the conditions of ESI-MS analysis is the production of alkali metal (Na and K) cationized adducts. The probability the of locations of Na cations in per-O-methylated compounds was proved by quantum chemical calculations, using the Jaguar program. The most probably position of alkali metal attachment is the carboxy group of the methoxycarbonyl C-5 group of the uronic acid unit. Characteristic cleavages vary according the kind of O-derivatization. In most cases they take place on the acidic part of the dimer and at the interglycosidic oxygen atom. As a result, the criteria for the differentiation of aldobiouronic and pseudoaldobiouronic acids derivatives were elucidated. 相似文献
997.
F López Arbeloa J Ba?uelos Prieto V Martínez Martínez T Arbeloa López I López Arbeloa 《Chemphyschem》2004,5(11):1762-1771
Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media. 相似文献
998.
Michael C. Bhm Joachim Schulte Rafael Ramírez 《International journal of quantum chemistry》2002,86(3):280-296
The absolute magnetic shieldings of benzene and ethylene have been theoretically studied under the conditions of thermal equilibrium, i.e., under explicit consideration of the nuclear degrees of freedom. For this purpose we have combined the Feynman path integral quantum Monte Carlo (PIMC) formalism with the gauge‐including atomic orbital (GIAO) approach in the Hartree–Fock (HF) approximation. The HF operator has been employed to derive the NMR parameters of the two hydrocarbons via an ensemble averaging over large sets of molecular configurations that are populated in thermal equilibrium. The nuclear fluctuations are responsible for a deshielding of the nuclei relative to the shieldings at the vibrationless minimum of the potential energy surface (PES). The influence of the nuclear degrees of freedom is largest for the isotropic part of the 13C shielding tensor. The theoretical results can be explained on the basis of simple geometrical considerations. The bond lengths in thermal equilibrium are larger than the bond lengths at the minimum of the PES. This length enhancement is the prerequisite for a deshielding of the nuclei in thermal equilibrium. The vibrational corrections of the nuclear magnetic resonance (NMR) parameters of benzene and ethylene are quantum driven; classical thermal degrees of freedom of the nuclei are of minor importance. Conceptual problems of theoretical studies of NMR parameters on the basis of a single molecular geometry are emphasized. The influence of the spatial uncertainty of the nuclei becomes decisive in molecules with light atoms. It is pointed out that the combination of the PIMC formalism with electronic Hamiltonians of state‐of‐the‐art quality renders possible accurate determinations of NMR parameters. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem 86: 280–296, 2002 相似文献
999.
A novel hybrid optimization method called quantum stochastic tunneling has been recently introduced. Here, we report its implementation within a new docking program called EasyDock and a validation with the CCDC/Astex data set of ligand-protein complexes using the PLP score to represent the ligand-protein potential energy surface and ScreenScore to score the ligand-protein binding energies. When taking the top energy-ranked ligand binding mode pose, we were able to predict the correct crystallographic ligand binding mode in up to 75% of the cases. By using this novel optimization method run times for typical docking simulations are significantly shortened. 相似文献
1000.
Mauricio Alcolea Palafox Nerea Iza Manuel Gil Jos Luís Núez 《International journal of quantum chemistry》2002,89(1):25-47
A quantum chemical study of several complex monocyclic 4‐benzoyl‐4‐phenyl‐β‐lactam derivatives was carried out using cyclobutane, azetidine, 2‐azetidinone, 1‐methyl‐2‐azetidinone, and 3‐methyl‐2‐azetidinone as model compounds. The optimum geometry was obtained for the different conformations. The planarity of the ring was discussed in terms of the influence of the substituents on the amide resonance. To better analyze the amide resonance and the activity of the β‐lactam ring, a vibrational study was also carried out. To examine the influence of solvent polarity on the carbonyl bands, the Fourier transform–infrared (FT‐IR) spectra of the β‐lactam monocyclic derivatives were recorded in CCl4, C6H6, and CHCl3 solutions. The normal vibrations of the β‐lactam ring in the model compounds were characterized and used in the analysis of the β‐ring of more complex derivatives. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 相似文献