Xiaogang Peng

Bio: Xiaogang Peng is an academic researcher from Zhejiang University. The author has contributed to research in topics: Quantum dot & Nanocrystal. The author has an hindex of 87, co-authored 205 publications receiving 52446 citations. Previous affiliations of Xiaogang Peng include Lawrence Berkeley National Laboratory & Jilin University.

Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals

Abstract: The extinction coefficient per mole of nanocrystals at the first exitonic absorption peak, e, for high-quality CdTe, CdSe, and CdS nanocrystals was found to be strongly dependent on the size of the nanocrystals, between a square and a cubic dependence. The measurements were carried out using either nanocrystals purified with monitored purification procedures or nanocrystals prepared through controlled etching methods. The nature of the surface ligands, the refractive index of the solvents, the PL quantum yield of the nanocrystals, the methods used for the synthesis of the nanocrystals, and the temperature for the measurements all did not show detectable influence on the extinction coefficient for a given sized nanocrystal within experimental error.

Shape control of CdSe nanocrystals

Abstract: Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties1,2 that depend sensitively on both size and shape3,4, and are of both fundamental and technological interest In contrast to the syntheses of zero-dimensional systems, existing preparations of one-dimensional systems often yield networks of tubes or rods which are difficult to separate5,6,7,8,9,10,11,12 And, in the case of optically active II–VI and III–V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects6,8,9,10 Thus, except for some metal nanocrystals13, there are no methods of preparation that yield soluble and monodisperse particles that are quantum-confined in two of their dimensions For semiconductors, a benchmark preparation is the growth of nearly spherical II–VI and III–V nanocrystals by injection of precursor molecules into a hot surfactant14,15 Here we demonstrate that control of the growth kinetics of the II–VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one This method should be useful, not only for testing theories of quantum confinement, but also for obtaining particles with spectroscopic properties that could prove advantageous in biological labelling experiments16,17 and as chromophores in light-emitting diodes18,19

Organization of 'nanocrystal molecules' using DNA

Abstract: PATTERNING matter on the nanometre scale is an important objective of current materials chemistry and physics. It is driven by both the need to further miniaturize electronic components and the fact that at the nanometre scale, materials properties are strongly size-dependent and thus can be tuned sensitively1. In nanoscale crystals, quantum size effects and the large number of surface atoms influence the, chemical, electronic, magnetic and optical behaviour2—4. 'Top-down' (for example, lithographic) methods for nanoscale manipulation reach only to the upper end of the nanometre regime5; but whereas 'bottom-up' wet chemical techniques allow for the preparation of mono-disperse, defect-free crystallites just 1–10 nm in size6–10, ways to control the structure of nanocrystal assemblies are scarce. Here we describe a strategy for the synthesis of'nanocrystal molecules', in which discrete numbers of gold nanocrystals are organized into spatially defined structures based on Watson-Crick base-pairing interactions. We attach single-stranded DNA oligonucleotides of defined length and sequence to individual nanocrystals, and these assemble into dimers and trimers on addition of a complementary single-stranded DNA template. We anticipate that this approach should allow the construction of more complex two-and three-dimensional assemblies.

Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor.

Abstract: High-quality colloidal semiconductor nanocrystals are nanometer-sized, single crystalline fragments of the corresponding bulk crystals, which have well-controlled size and size distribution and are dispersible in desired solvents/media. Recently, semiconductor nanocrystals are of great interest for both fundamental research and technical applications, 1-8 due to their strong size dependent properties and excellent chemical processibility. Synthesis of highquality semiconductor nanocrystals has been playing a critical role in this very active field. 1,9-15 As the most developed system in terms of synthesis, 1,9,10,15high-quality CdSe nanocrystals with nearly monodisperse size and shape are in active industrial development for biological labeling reagents. 5,6 Since Murray et al. 15 reported the synthesis of high quality cadmium chalcogenides nanocrystals using dimethyl cadmium (Cd(CH 3)2) as the cadmium precursor, the synthesis of CdSe nanocrystals using this precursor has been well developed. 1,9,10In comparison, the synthesis of CdTe and CdS15,16are not as advanced. For instance, there is no method to controllably vary the shape of CdTe and CdS nanocrystals. Cd(CH3)2 is extremely toxic, pyrophoric, expensive, unstable at room temperature, and explosive at elevated temperatures by releasing large amount of gas. Due to these reasons, the Cd(CH 3)2related schemes require very restricted equipments and conditions and are not suited for large-scale synthesis. In this paper, we will prove that Cd(CH3)2 can be replaced by CdO. Surprisingly, this new synthetic scheme works significantly better than the Cd(CH3)2-related ones. Without any size-sorting, the quality of quantum-confined dots and rods (quantum dots and quantum rods) of all cadmium chalcognides formed by the new method is comparable to that of the best CdSe nanocrystals reported in the literature. The new scheme is reproducible and simple and thus can be readily scaled up for industrial production. Recently, we identified that Cd(CH 3)2 decomposes in hot trioctylphosphine oxide (TOPO) and generates insoluble metallic precipitate. 9 With a strong ligand, either hexylphosphonic acid (HPA) or tetradecylphosphonic acid (TDPA), Cd(CH 3)2 is immediately converted into cadmium HPA/TDPA complex (Cd HPA/Cd-TDPA) if the cadmium to HPA/TDPA ratio is lower than 1. After the formation of the complex, an injection of Se dissolved in tributylphosphine (TBP) generates high-quality CdSe nanocrystals. This result implies that Cd(CH 3)2 may not be necessary, if we can generate the complex by other means. We first synthesized and purified Cd -HPA from CdCl 2 or Cd(CH3)2. High-quality CdSe nanocrystals were indeed yielded from this complex. This success encouraged us to develop a one-pot synthesis which does not require separated preparation of cadmium complex. We failed to make high-quality CdSe nanocrystals using CdCl2 by the one-pot approach although CdCl 2 can be dissolved in the reaction mixture at elevated temperatures. In contrast, CdO works very well for the one-pot approach. We think this is due to the low stability of CdO relative to phosphonic acids, compared to that of CdCl 2. Experimentally, CdO, TOPO, and HPA/TDPA were loaded in a three-neck flask. At about 300 °C, reddish CdO powder was dissolved and generated a colorless homogeneous solution. Introducing tellurium, selenium, and sulfur stock solutions yields high quality nanocrystals. 17 The samples for all of the measurements shown in this paper are directly from synthesis without any size separation. The growth kinetics of nanocrystals grown by the new approach possesses a pattern similar to that of the best CdSe nanocrystals formed by the Cd(CH3)2 approach (Figure 1). 10 Figure 1 and Figure 2 further reveal that the size of all three kinds of nanocrystals can be close to monodisperse, represented by the sharp absorption peaks if the growth stops in the “focusing of size distribution” regime. 10 Transmission electron microscopy (TEM) measurements indicate that these nanocrystals have very narrow distribution. The relative standard deviation of the size of the nanocrystals shown in Figure 3 (top) is about 10%. The high crystallinity of these wurtzite nanocrystals was confirmed by X-ray powder diffraction. For this CdO approach, the size of relatively monodisperse CdSe nanocrystals can be continuously tuned down to the sizes with the first absorption peak at 440 nm (see the first absorption spectrum in Figure 1). Relatively monodisperse CdSe nanocrystals with the first exciton absorption peak below 480 nm are difficult to synthesize directly with the existing Cd(CH 3)2-related approach. 10,18 (1) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature2000, 404, 59-61. (2) Heath, J. R. (editor). Acc.f Chem. Res. 1999. (3) Alivisatos, A. P.Science1996, 271, 933-937. (4) Huynh, W.; Peng, X.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923927. (5) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (6) Chan, W. C. W.; Nie, S. M. Science1998, 281, 2016-2018. (7) Schlamp, M. C.; Peng, X. G.; Alivisatos, A. P. J. Appl. Phys.1997, 82, 5837-5842. (8) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J. Appl. Phys.1998, 83, 7965-7974. (9) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. , in revision. (10) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344. (11) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J Am. Chem. Soc. 1993, 115, 8706-8715. (12) Nozik, A. J.; Micic, O. I.MRS Bull.1998, 23, 24-30. (13) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019-7029. (14) Dabbousi, B. O.; RodriguezViejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475. (15) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 76657673. (16) Mikulee, F.; Ph.D. Thesis, MIT, Boston, 1998. (17) A typical synthesis for CdTe nanocrystals: 0.0514 g of CdO, 0.2232 g of TDPA and 3.7768 g of TOPO were loaded into a 25 mL flask. The mixture was heated to 300 -320 °C under Ar flow, and CdO was dissolved in TDPA and TOPO. The temperature of the solution was cooled to 270 °C, tellurium stock solution (0.0664 g of tellurium powder dissolved i n 2 g of TOP) was injected. After injection, nanocrystals grew at 250 °C to reach desired size. (18) Bawendi, M. G. Private communication. Figure 1. Temporal evolution of size and size distribution of CdTe, CdSe, and CdS nanocrystals studied by UV -vis. 183 J. Am. Chem. Soc. 2001,123,183-184

Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility

Abstract: The synthesis of epitaxially grown, wurtzite CdSe/CdS core/shell nanocrystals is reported Shells of up to three monolayers in thickness were grown on cores ranging in diameter from 23 to 39 A Shell growth was controllable to within a tenth of a monolayer and was consistently accompanied by a red shift of the absorption spectrum, an increase of the room temperature photoluminescence quantum yield (up to at least 50%), and an increase in the photostability Shell growth was shown to be uniform and epitaxial by the use of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and optical spectroscopy The experimental results indicate that in the excited state the hole is confined to the core and the electron is delocalized throughout the entire structure The photostability can be explained by the confinement of the hole, while the delocalization of the electron results in a degree of electronic accessibility that makes these nanocrystals

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

Abstract: Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Spintronics: a spin-based electronics vision for the future.

Abstract: This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

Spintronics: Fundamentals and applications

Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

Semiconductor Nanocrystals as Fluorescent Biological Labels

Abstract: Semiconductor nanocrystals were prepared for use as fluorescent probes in biological staining and diagnostics. Compared with conventional fluorophores, the nanocrystals have a narrow, tunable, symmetric emission spectrum and are photochemically stable. The advantages of the broad, continuous excitation spectrum were demonstrated in a dual-emission, single-excitation labeling experiment on mouse fibroblasts. These nanocrystal probes are thus complementary and in some cases may be superior to existing fluorophores.