Near-edge and extended x-ray-absorption fine-structure measurements from a wide variety of oxidized Si nanocrystals and H-passivated porous Si samples, combined with electron microscopy, ir absorption, forward recoil scattering, and luminescence emission data, provide a consistent structural picture of the species responsible for the luminescence observed in these systems. For porous Si samples whose luminescence wavelengths peak in the visible region, i.e. , at (700 nm, their mass-weighted-average structures are determined here to be particles (not wires) whose short-range character is crystalline and 0 whose dimensionstypically (15 Aare significantly smaller than previously reported or proposed.Results are also presented which demonstrate that the observed visible luminescence is not related to either a photo-oxidized Si species in porous Si or an interfacial suboxide species in the Si nanocrystals. The structural and compositional findings reported here depend only on sample luminescence behavior, not on how the luminescent particles are produced, and thus have general implications in assigning quantum confinement as the mechanism responsible for the visible luminescence observed in both nanocrystalline and porous silicon.

The possibility induction of light emission from silicon, an indirect bandgap material in which radiative transitions are unlikely, raises several interesting and technologically important possibilities, especially the fabrication of a truly integrated optoelectronic microchip. In this article, the natural considerations that constrain silicon from emitting light efficiently are examined, as are several engineered solutions to this limitation. These include intrinsic and alloy-induced luminescence; radiatively active impurities; quantum-confined structures, including zone folding and the recent developments in porous silicon; and a hybrid approach, the integration of direct bandgap materials onto silicon.

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Prominent resonance Raman and photoluminescence spectroscopic differences between AA′ and AB stacked bilayer molybdenum disulfide (MoS2) grown by chemical vapor deposition are reported. Bilayer MoS2 islands consisting of the two stacking orders were obtained under identical growth conditions. Resonance Raman and photoluminescence spectra of AA′ and AB stacked bilayer MoS2 were obtained on Au nanopyramid surfaces under strong plasmon resonance. Both resonance Raman and photoluminescence spectra show distinct features indicating clear differences in interlayer interaction between these two phases. The implication of these findings on device applications based on spin and valley degrees of freedom will be discussed.

The general theoretical de®nition of an insulator is a material in which the conductivity vanishes at the absolute zero of temperature. In classical insulators, such as materials with a band gap, vanishing conductivities lead to diverging resistivities. But other insulators can show more complex behaviour, particularly in the presence of a high magnetic ®eld, where different components of the resistivity tensor can display different behaviours: the magnetoresistance diverges as the temperature approaches absolute zero, but the transverse (Hall) resistance remains ®nite. Such a system is known as a Hall insulator 1 . Here we report experimental evidence for a quantized 2 Hall insulator in a two-dimensional electron systemÐcon®ned in a semiconductor quantum well. The Hall resistance is quantized in the quantum unit of resistance h/e 2 , where h is Planck's constant and e the electronic charge. At low ®elds, the sample reverts to being a normal Hall insulator.Experimentally, the identi®cation of an insulating phase is based on extrapolating the measured magnetoresistance, r xx (T), at ®nite temperature (T) to T 0. This is always an ambiguous process. However, when r xx is exponentially increasing as T ! 0, the state is usually considered to be an insulator. Unfortunately, the divergent r xx seriously hinders the determination of the Hall resistivity, r xy , as even small Hall-contact misalignment will result in a large overriding signal from the diverging r xx . It is possible, to a certain degree, to circumvent this dif®culty by symmetrizing the measurement. This can be achieved by reversing the magnetic ®eld (B) orientation, as the contribution of r xx is symmetric in B as opposed to antisymmetric for r xy . The effectiveness of this procedure is demonstrated in the inset of Fig. 1, where we show measurements made in a twodimensional hole system con®ned in a 150-A Ê -thick strained Ge layer sandwiched in between Si 0.4 Ge 0.6 layers with boron modulation doping. More details of this system are given in ref. 3. The Hall resistances obtained for the two opposite B-®eld directions are in dotted lines and the average, that is r xy , is shown with a solid line. All Hall resistivities discussed here are obtained by this method.We now turn to discuss our results, where our ®rst task is to identify the different phases. The transition between insulating and quantum Hall phases can be characterized by a critical B-®eld value, for which r xx is T-independent and where the derivative of the Tdependence changes sign on each side of the transition. By plotting r xx at two different values of T, we can therefore extract the transition points. In Fig. 1 we have plotted r xy together with r xx as a function of B. With increasing B, transition points at B 2:2 T and at B B C 6:06 T can be identi®ed from the crossing of the two r xx curves obtained at different values of T. Between these transitions we have the usual quantum Hall state, which is bordered on both sides by insulators. For clarity, r xx is normalized to r c r xx B C 4 ...

We present a magnetotransport study of a disordered two-dimensional hole system in a strained Ge quantum well. As the magnetic field is increased, a clear transition from a low magnetic field insulator to the n 1 quantum Hall state at the lowest density range (controlled by a gate), and to the n 3 state at higher densities, is observed. We find that these transitions are characterized by a new universality: At the critical point, the diagonal and Hall resistivities are equal, within experimental uncertainty. These results are in conflict with the "floating" scenario suggested by Khmel'nitzkii [JETP Lett. 38, 552 (1983)] and Laughlin [Phys. Rev. Lett. 52, 2304(1984]. [S0031-9007(97)02740-3]

The effect of impurity coimplantation in MeV erbium-implanted silicon is studied. A significant increase in the intensity of the 1.54-μm Er3+ emission was observed for different coimplants. This study shows that the Er3+ emission is observed if erbium can form an impurity complex in silicon. The influence of these impurities on the Er3+ photoluminescence spectrum is demonstrated. Furthermore we show the first room-temperature photoluminescence spectrum of erbium in crystalline silicon.

X-ray absorption measurements from H-passivated porous Si and from oxidized Si nanocrystals, combined with electron microscopy, ir absorption, a recoil, and luminescence emission data, provide a consistent structural picture of the species responsible for the visible luminescence observed in these samples. The mass-weighted average structures in por-Si are particles, not wires, with dimensions significantly smaller than previously reported or proposed. PACS numbers: 78.70.Dm, 61.10.Lx, 61.46.+w, 78.55. -m The novel properties and possible utility of visible room-temperature luminescence from anodically grown porous silicon (por-Si) have generated intense study [1], from which a growing consensus has emerged for explaining the luminescence with quantum-confined structures [2,3]. (Luminescence from por-Si passivated with 0 rather than H [4] has ruled out a SiH"species, while the absence of Si-0 bonding in x-ray absorption data from por-Si [5] has ruled out siloxenes. ) There remains, however, a basic lack of knowledge regarding the dimensions and shape of the species actually responsible for the optical activity, largely because the material is inhomogeneous, but also because there is little direct structural information from the region lying within the penetration depth of the photoexciting radiation. Even for 80% porosity samples excited by -350-400-nm light (where the luminescence is a maximum [5,6]), this depth is & 5000 A [7] and thus not well suited for standard microscopy or diffraction techniques from as-prepared material. Moreover, Si structures & 20 A, should they be significant, are beyond practical detection with these methods. Added to these experimental limitations are theoretical uncertainties in correlating band gap with Si size, where nanoscale particle or wire dimensions for a given gap vary by more than 100% [1,8]. %e report on x-ray absorption measurements from a series of oxidized Si nanocrystals, whose shapes and sizes are known, and from a variety of anodically grown (Hpassivated) por-Si samples. The data, combined with luminescence emission measurements from each of the systems, establish new and unexpectedly smaller values for the average size of Si structures contained within optically relevant depths of por-Si. In addition, the general importance of extended wire shapes is ruled out. Our results have significant implications for describing the origin of visible photoluminescence from por-Si. The Si E-edge absorption measurements were performed at the National Synchrotron Light Source using the AT&T X158 beam line [9] and InSb(111) monochromating crystals. Samples were kept at 77 K to minimize thermal disorder effects [10]. All data were obtained with total electron yield detection, whose effective sampling depth in 80%%ue porosity Si is & 2500 A [11]. A variety of por-Si samples prepared under very different conditions [12] were studied with transmission electron microscopy (TEM), x-ray and ir absorption, tt recoil, and luminescence excitation and emission spectroscopies. Air exposure of ...