Photoluminescence properties of surface-oxidized Ge nanocrystals: Surface localization of excitons

Morphological and Surface-State Challenges in Ge Nanoparticle Applications

The intrinsic properties of Ge in tandem with advances in its nanostructuring have resulted in its increased attention in a variety of fields as an alternative to traditional group 12-14 and 14-16 nanoparticles (NPs). The small band gap and size-dependent development of the optical properties in tandem with their good charge transport properties make Ge NPs a suitable material for optoelectronic devices. The low toxicity of Ge, together with its IR photoluminescence (PL) that overlaps with desirable biological optical windows used for tissue imaging, allows the exploitation of these materials in the field of bioimaging and as drug carriers. In addition, the ability of germanium to both exhibit high mechanical stability in its NP form and alloy with lithium and sodium metals has led to it being a highly attractive material for next-generation lithium ion and beyond-lithium batteries. While it is attracting considerable attention in a variety of areas, research on Ge NPs is still relatively nascent. Fundamental aspects of this material, such as its Bohr radius and the origin of different observed PLs, are still under debate. Moreover, the ability to produce Ge NPs with controlled dimensions and morphology is not yet as mature as for other classes of nanomaterials. In this review, the mechanisms and origins of these properties will be introduced, which we then relate to specific applications presented in the literature.

One-Pot Synthesis of Gel Glass Embedded with Luminescent Silicon Nanoparticles

Preparation of highly luminescent glasses involves expensive and complicated processes and usually requires high temperature. In this work, we show that luminescent silicon (Si) nanoparticle (NP)- embedded silicate gel glasses can be developed under near-ambient conditions by a remarkably simple, one-pot strategy, without using any sophisticated instrumentation or technique. Simultaneous hydrolysis and reduction of (3-aminopropyl)triethoxysilane leads to the formation of colloidal Si nanocrystals that can be transformed to a glassy phase upon slow evaporation followed by freezing. Structural investigations reveal the formation of a sodium silicate gel glass framework having discernible shear bands, along with embedded Si NPs. High photoluminescence quantum yield (ca. 35-40%), low glass-transition temperature ( T_g ≈ 66-73 °C), strain-tolerant mechanical stability, and inexpensive preparation make the glass attractive for applications as display materials and photonic converters.

Effect of Ge Nanocrystals on 1.54 μm Photoluminescence Enhancement in Er₂O₃:ZnO and Ge Co-Sputtered Films

Photoluminescence (PL) of Er and Ge co-doped ZnO films synthesized by radio frequency magnetron co-sputtering was investigated. X-ray diffraction (XRD) patterns showed that the annealing process at 400–800 °C led to the formation of nanocrystal (nc) Ge. Samples containing nc-Ge showed a strong visible PL with a peak at 582–593 nm, which was consistent with the calculated energy of the exciton of the ~5 nm-sized nc-Ge, according to the quantum confinement effect. The formation of nc-Ge could greatly enhance the 1.54 μm emission, and it is considered that the 1.54 μm PL enhancement may come from a joint effect of both the energy transfer from nc-Ge to Er³⁺ and the local environment change of Er³⁺.

Carrier trapping and confinement in Ge nanocrystals surrounded by Ge3N4

We investigated the optical properties of Ge nanocrystals surrounded by Ge₃N₄. The broad emission ranging from infrared to blue is due to the dependence on the crystal size and preparation methods. Here, we report high resolution Photoluminescence (PL) attributed to emission from individual Ge nanocrystals (nc-Ge) spatially resolved using micro-photoluminescence and detailed using temperature and power-dependent photoluminescence studies. The measured peaks are shown to behave with excitonic characteristics and we argue that the spread of the nc-Ge peaks in the PL spectrum is due to different confinement energies arising from the variation in size of the nanocrystals.

Direct Evaluation of the Quantum Confinement Effect in Single Isolated Ge Nanocrystals

To address the yet open question regarding the nature of quantum confinement in Ge nanocrystals (Ge NCs) we employed scanning tunneling spectroscopy to monitor the electronic structure of individual isolated Ge NCs as a function of their size. The (single-particle) band gaps extracted from the tunneling spectra increase monotonically with decreasing nanocrystal size, irrespective of the capping ligands, manifesting the effect of quantum confinement. Band-gap widening of ∼1 eV with respect to the bulk value was observed for Ge-NCs 3 nm in diameter. The picture emerging from comparison with theoretical calculations and other experimental results is discussed.

Template-directed assembly of metal-chalcogenide nanocrystals into ordered mesoporous networks

Although great progress in the synthesis of porous networks of metal and metal oxide nanoparticles with highly accessible pore surface and ordered mesoscale pores has been achieved, synthesis of assembled 3D mesostructures of metal-chalcogenide nanocrystals is still challenging. In this work we demonstrate that ordered mesoporous networks, which comprise well-defined interconnected metal sulfide nanocrystals, can be prepared through a polymer-templated oxidative polymerization process. The resulting self-assembled mesostructures that were obtained after solvent extraction of the polymer template impart the unique combination of light-emitting metal chalcogenide nanocrystals, three-dimensional open-pore structure, high surface area, and uniform pores. We show that the pore surface of these materials is active and accessible to incoming molecules, exhibiting high photocatalytic activity and stability, for instance, in oxidation of 1-phenylethanol into acetophenone. We demonstrate through appropriate selection of the synthetic components that this method is general to prepare ordered mesoporous materials from metal chalcogenide nanocrystals with various sizes and compositions.

Synthesis and structure of free-standing germanium quantum dots and their application in live cell imaging

Colloidally synthesized free-standing Ge qdots with a unique core–shell structure were demonstrated to be a viable bio-imaging probe.

Structural and optical characterization of self-assembled Ge nanocrystal layers grown by plasma-enhanced chemical vapor deposition

We present a structural and optical study of solid-state dispersions of Ge nanocrystals prepared by plasma-enhanced chemical vapor deposition. Structural analysis shows the presence of nanocrystalline germanium inclusions embedded in an amorphous matrix of Si-rich SiO(2).Optical characterization reveals two prominent emission bands centered around 2.6 eV and 3.4 eV, and tunable by excitation energy. In addition, the lower energy band shows an excitation power-dependent blue shift of up to 0.3 eV. Decay dynamics of the observed emission contains fast (nanosecond) and slow (microseconds) components, indicating contributions of several relaxation channels. Based on these material characteristics, a possible microscopic origin of the individual emission bands is discussed.

Treatment of TiO2 with COOH-functionalized germanium nanoparticles to enhance the photocurrent of dye-sensitized solar cells

A dye-sensitized solar cell (DSSC) containing a TiO(2) film treated with COOH-functionalized germanium nanoparticles (Ge-COOH Nps) exhibited a higher short-circuit photocurrent density (J(sc); 15.4 mA cm(-2)) compared to the corresponding untreated DSSC (13.4 mA cm(-2)) using N719 and a 12 μm thick TiO(2) film at 100 mW cm(-2). The amount of N719 attached to the treated TiO(2) film was 21% greater than that attached to the untreated TiO(2) film. Enhancement of the J(sc) value by 15% was attributed mostly to an intramolecular charge transfer from N719 attached to the Ge-COOH Nps to the TiO(2) conduction band through the Ge-COOH Nps.

Effect of ion implantation energy for the synthesis of Ge nanocrystals in SiN films with HfO2/SiO2 stack tunnel dielectrics for memory application

Ge nanocrystals (Ge-NCs) embedded in SiN dielectrics with HfO2/SiO2 stack tunnel dielectrics were synthesized by utilizing low-energy (≤5 keV) ion implantation method followed by conventional thermal annealing at 800°C, the key variable being Ge+ ion implantation energy. Two different energies (3 and 5 keV) have been chosen for the evolution of Ge-NCs, which have been found to possess significant changes in structural and chemical properties of the Ge+-implanted dielectric films, and well reflected in the charge storage properties of the Al/SiN/Ge-NC + SiN/HfO2/SiO2/Si metal-insulator-semiconductor (MIS) memory structures. No Ge-NC was detected with a lower implantation energy of 3 keV at a dose of 1.5 × 1016 cm-2, whereas a well-defined 2D-array of nearly spherical and well-separated Ge-NCs within the SiN matrix was observed for the higher-energy-implanted (5 keV) sample for the same implanted dose. The MIS memory structures implanted with 5 keV exhibits better charge storage and retention characteristics compared to the low-energy-implanted sample, indicating that the charge storage is predominantly in Ge-NCs in the memory capacitor. A significant memory window of 3.95 V has been observed under the low operating voltage of ± 6 V with good retention properties, indicating the feasibility of these stack structures for low operating voltage, non-volatile memory devices.

Direct-bandgap luminescence at room-temperature from highly-strained Germanium nanocrystals

Efficient room-temperature luminescence at optical telecommunication wavelengths and originating from direct band-to-band recombination has been observed in tensile-strained germanium nanocrystals synthesized by mechanical grinding techniques. Selected area electron diffraction, micro-Raman and optical-absorption spectroscopy measurements indicate high tensile-strains while combined photoluminescence spectroscopy, excitation-power evolution and time-resolved measurements suggest direct band-to-band recombination. Such band-engineered germanium nanocrystals offer great possibilities for silicon-photonics integration due to their superb light-emission properties, facile fabrication and compatibility with standard microelectronic processes.