Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.

Emerging Photoluminescence in Monolayer MoS2

Nanomaterials that can cir culate in the body hold great potential to diagnose and treat disease 1‐4 . For such applications, it is important that the nanomaterials be harmlessly eliminated from the body in a reasonable period of time after they carry out their diagnostic or therapeutic function. Despite efforts to improve their targeting efficiency, significant quantities of systemically administered nanomaterials are cleared by the mononuclear phagocytic system before finding their targets, increasing the likelihood of unintended acute or chronic toxicity. However, there has been little effort to engineer the self-destruction of errant nanoparticles into non-toxic, systemically eliminated products. Here, we present luminescent porous silicon nanoparticles (LPSiNPs) that can carry a drug payload and of which the intrinsic near-infrared photoluminescence enables monitoring of both accumulation and degradation in vivo. Furthermore, in contrast to most optically active nanomaterials (carbon nanotubes, gold nanoparticles and quantum dots), LPSiNPs self-destruct in a mouse model into renally cleared components in a relatively short period of time with no evidence of toxicity. As a preliminary in vivo application, we demonstrate tumour imaging using dextran-coated LPSiNPs (D-LPSiNPs). These results demonstrate a new type of multifunctional nanostructure with a low-toxicity degradation pathway forinvivo applications. The in vivo use of nanomaterials as therapeutic and diagnostic agents is of intense interest owing to their unique properties such as large specific capacity for drug loading 2 , strong superparamagnetism 3 ,efficientphotoluminescence 1,5 ordistinctive Raman signatures 4 , among others. Materials with sizes in the range of 20200nm can avoid renal filtration, leading to prolonged resi

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Biodegradable luminescent porous silicon nanoparticles for in vivo applications.

Silicon has long been established as the material of choice for the microelectronics industry. This is not yet true in photonics, where the limited degrees of freedom in material design combined with the indirect bandgap are a major constraint. Recent developments, especially those enabled by nanoscale engineering of the electronic and photonic properties, are starting to change the picture, and some silicon nanostructures now approach or even exceed the performance of equivalent direct-bandgap materials. Focusing on two application areas, namely communications and photovoltaics, we review recent progress in silicon nanocrystals, nanowires and photonic crystals as key examples of functional nanostructures. We assess the state of the art in each field and highlight the challenges that need to be overcome to make silicon a truly high-performing photonic material.

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Silicon nanostructures for photonics and photovoltaics

Crystalline silicon is the most important semiconductor material in the electronics industry. However, silicon has poor optical properties because of its indirect bandgap, which prevents the efficient emission and absorption of light. The energy structure of silicon can be manipulated through quantum confinement effects, and the excitonic emission from silicon nanocrystals increases in intensity and shifts to shorter wavelengths (a blueshift) as the size of the nanocrystals is reduced. Here we report experimental evidence for a short-lived visible band in the photoluminescence spectrum of silicon nanocrystals that increases in intensity and shifts to longer wavelengths (a redshift) with smaller nanocrystal sizes. This higher intensity indicates an increased quantum efficiency, which for 2.5-nm-diameter nanocrystals is enhanced by three orders of magnitude compared to bulk silicon. We assign this band to the radiative recombination of non-equilibrium electron-hole pairs in a process that does not involve phonons.

Red spectral shift and enhanced quantum efficiency in phonon-free photoluminescence from silicon nanocrystals

In this review, the emerging roles of group IV nanoparticles including silicon, diamond, silicon carbide, and germanium are summarized and discussed from the perspective of biologists, engineers, and medical practitioners. The synthesis, properties, and biological applications of these new nanomaterials have attracted great interest in the past few years. They have gradually evolved into promising biomaterials due to their innate biocompatibility; toxic ions are not released when they are used in vitro or in vivo, and their wide fluorescence spectral regions span the near-infrared, visible, and near-ultraviolet ranges. Additionally, they generally have good resistance against photobleaching and have lifetimes on the order of nanoseconds to microseconds, which are suitable for bioimaging. Some of the materials possess unique mechanical, chemical, or physical properties, such as ultrachemical and thermal stability, high hardness, high photostability, and no blinking. Recent data have revealed the superiority of these nanoparticles in biological imaging and drug delivery.

Group IV nanoparticles: synthesis, properties, and biological applications.

Silicon dominates the electronics industry, but its poor optical properties mean that III–V compound semiconductors are preferred for photonics applications. Photoluminescence at visible wavelengths was observed from porous Si at room temperature in 1990, but the origin of these photons (do they arise from highly localized defect states or quantum confinement effects?) has been the subject of intense debate ever since. Attention has subsequently shifted from porous Si to Si nanocrystals, but the same fundamental question about the origin of the photoluminescence has remained. Here we show, based on measurements in high magnetic fields, that defects are the dominant source of light from Si nanocrystals. Moreover, we show that it is possible to control the origin of the photoluminescence in a single sample: passivation with hydrogen removes the defects, resulting in photoluminescence from quantum-confined states, but subsequent ultraviolet illumination reintroduces the defects, making them the origin of the light again.

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Classification and control of the origin of photoluminescence from Si nanocrystals

A K- and Q-band electron spin resonance study, combining two detection modes, on structures composed of Si nanoparticles (∼2nm across) embedded in a SiO2 matrix obtained by annealing of SiO∕SiO2 superstructures unveils the observed principal Si dangling bond signal as solely comprised of the intrinsic Pb(0) and Pb1 Si∕SiO2 interface defect spectra, of comparable densities. Based on independent particle density counting, about one Pb-type defect is found at every 1.4 Si nanoparticles. Relying on the known properties of Pb-type defects in standard microscopic Si∕SiO2, the data would comply with Si nanocrystallites with morphology, schematically, of [100] truncated (111) octahedrons. Both types and densities of observed point defects indicate a structure of quality much alike that of standard thermal Si∕SiO2.

Paramagnetic point defects at SiO2/nanocrystalline Si interfaces

We have used photoluminescence in pulsed (⩽50T) and dc (⩽12T) magnetic fields to investigate the influence of substrate orientation and growth interruption (GI) on the electronic properties of InAs∕GaAs quantum dots, grown by molecular beam epitaxy at 480°C. Dot formation is very efficient on the (100) substrate: electronic confinement is already strong without GI and no significant change in confinement is observed with GI. On the contrary, for the (311)B substrate strong confinement of the charges only occurs after a GI is introduced. When longer GIs are applied the dots become higher.

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Magnetophotoluminescence study of the influence of substrate orientation and growth interruption on the electronic properties of InAs /GaAs quantum dots

S. Godefroo

Papers

Classification and control of the origin of photoluminescence from Si nanocrystals

Paramagnetic point defects at SiO2/nanocrystalline Si interfaces

Magnetophotoluminescence study of the influence of substrate orientation and growth interruption on the electronic properties of InAs /GaAs quantum dots