The transport of photoinjected charges in disordered organic films is often interpreted using a formula based on a Gaussian disorder model (GDM) that neglects spatial correlations due to chargedipole interactions, even though such correlations have recently been shown to explain the universal electric field dependence observed in these systems. Based on extensive computer simulations of a 3D disorder model that includes such correlations, we present a new formula for analyzing experiments that accurately describes transport in these materials. [S0031-9007(98)07626-1] PACS numbers: 73.50.Yg, 72.10.Bg, 72.80.LeRecent efforts by a number of workers [1-7] have increased our understanding of nearly universal features of photoinjected charge transport in many disordered organic materials, including molecularly doped polymers [8,9], low molecular weight organic glasses [10,11], and certain polyconjugated polymers [12,13]. In particular, it is now recognized that the Poole-Frenkel (PF) dependence [8][9][10][11][12], m~exp͑gof the drift mobility m on electric field E observed in these materials results from slowly varying spatial fluctuations in the potential energy of a charge migrating through the material. Such energetic fluctuations can arise [1] from a random distribution of molecules in the medium possessing permanent electric dipole moments; a carrier's interaction with the latter provides a significant contribution U d to the total site energy. More importantly, the energy correlation function [1,3] C͑r͒ ͗U d ͑0͒U d ͑r͒͘ ϳ s 2 d a͞r (2) decays very slowly with intersite separation r. Here, s d ͗U 2 d ͘ 1͞2 is the rms width of the dipolar energetic disorder, and a is a minimal charge-dipole separation. In a previous Letter [3], an analytical result equivalent to (1) was derived for carriers diffusing along one spatial dimension through a medium with correlations as in (2). This same behavior was also observed in 3D charge transport simulations [4]. Moreover, very recent studies on both 1D and 3D systems suggest that this mechanism producing PF behavior is stable under additional sources of disorder less correlated than those that arise from dipoles [5,6], and indicate that the PF factor g in (1) is insensitive to all but the dipolar component of the disorder.These recent advances raise questions regarding the way materials have been experimentally characterized in the past. Most measurements in the last decade have been interpreted using an uncorrelated Gaussian disorder model, developed and extensively studied by Bässler and co-workers prior to the recent recognition of the importance of spatial correlations [14]. In the GDM, transport occurs through hops among localized states characterized by a Gaussian distribution of site energies, with hopping rates obeying an asymmetric detailed balance relation [14]. Numerical simulations capture well many features of experiment; its Gaussian density of states (DOS) leads to a temperature dependence ln m~2͑T 0 ͞T͒ 2 routinely observed, and the GDM reproduces low temperature...

Single photon emitters (SPEs) in hexagonal boron nitride (hBN) have garnered significant attention over the last few years due to their superior optical properties. However, despite the vast range of experimental results and theoretical calculations, the defect structure responsible for the observed emission has remained elusive. Here, by controlling the incorporation of impurities into hBN via various bottom-up synthesis methods and directly through ion implantation we provide direct evidence that the visible SPEs are carbon related. Room temperature optically detected magnetic resonance (ODMR) is demonstrated on ensembles of these defects. We perform ion implantation experiments and confirm that only carbon implantation creates SPEs in the visible spectral range. Computational analysis of the simplest 12 carbon-containing defect species suggest the negatively charged V B C N − defect as a viable 53 candidate and predict that out-of-plane deformations make the defect environmentally sensitive. 54Our results resolve a long-standing debate about the origin of single emitters at the visible range 55 grown epitaxially. 28, 29 The energy detuning between the ZPL of the ensemble and phonon sideband (PSB) peak is ~176 meV on average (Extended Data Fig. 1). 30, 31 X-ray photoelectron spectroscopy (XPS) was used to quantify the incorporation of carbon (Extended Data Fig. 2). Figure 1b(c) demonstrate a near linear correlation between C-B (C-N) bonding and increasing TEB flux, with C-B bonding being roughly an order of magnitude more prevalent than C-N bonding. Preferential formation of C-B bonds follows logically from noting the B species are introduced with three pre-existing bonds to C. PL intensity of the resulting ensemble emission likewise displays a linear correlation with carbon concentration (Extended Data Fig. 3). Based on these results, we advance that the SPE emission at ~580 nm in hBN is likely to originate from a carbon-related defect complex. Figure 1-Photoluminescence from MOVPE hBN Samples. a. MOVPE hBN grown with increasing flow rates of triethyl borane (TEB). As TEB flow increases, the fluorescence of SPE ensembles increases. b. Percentage of B-C bonding with increasing TEB flow evaluated by XPS. c. Percentage of N-C bonding with increasing TEB flow evaluated by XPS. d. Room temperature ODMR displayed as relative contrast, spin-dependent variation in photoluminescence (∆PL/PL), observed from the ~585 nm ensemble emission of MOPVE hBN (TEB 60) at applied fields of 19, 24, and 29 mT respectively. e.

Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. However, the intrinsic optical properties of monolayer boron nitride remain largely unexplored. In particular, the theoretically expected crossover to a direct-gap in the limit of the single monolayer is presently not confirmed experimentally. Here, in contrast to the technique of exfoliating few-layer 2D hexagonal boron nitride, we exploit the scalable approach of high-temperature molecular beam epitaxy to grow high-quality monolayer boron nitride on graphite substrates. We combine deep-ultraviolet photoluminescence and reflectance spectroscopy with atomic force microscopy to reveal the presence of a direct gap of energy 6.1 eV in the single atomic layers, thus confirming a crossover to direct gap in the monolayer limit.

We investigate the opto-electronic properties of hexagonal boron nitride grown by high temperature plasma-assisted molecular beam epitaxy. We combine atomic force microscopy, spectroscopic ellipsometry, and photoluminescence spectroscopy in the deep ultraviolet to compare the quality of hexagonal boron nitride grown either on sapphire or highly oriented pyrolytic graphite. For both substrates, the emission spectra peak at 235 nm, indicating the high optical quality of hexagonal boron nitride grown by molecular beam epitaxy. The epilayers on highly oriented pyrolytic graphite demonstrate superior performance in the deep ultraviolet (down to 210 nm) compared to those on sapphire. These results reveal the potential of molecular beam epitaxy for the growth of hexagonal boron nitride on graphene, and more generally, for fabricating van der Waals heterostructures and devices by means of a scalable technology. LETTEROriginal content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

We demonstrate direct epitaxial growth of high-quality hexagonal boron nitride (hBN) layers on graphite using high-temperature plasma-assisted molecular beam epitaxy. Atomic force microscopy reveals mono- and few-layer island growth, while conducting atomic force microscopy shows that the grown hBN has a resistance which increases exponentially with the number of layers, and has electrical properties comparable to exfoliated hBN. X-ray photoelectron spectroscopy, Raman microscopy and spectroscopic ellipsometry measurements on hBN confirm the formation of sp2-bonded hBN and a band gap of 5.9 ± 0.1 eV with no chemical intermixing with graphite. We also observe hexagonal moiré patterns with a period of 15 nm, consistent with the alignment of the hBN lattice and the graphite substrate.

Experimental evidence of population inversion and amplified spontaneous emission was found for Si nanocrystallites embedded in SiO2 surrounding under pumping with 5 ns light pulses at 380, 400, and 500 nm. As an important property, our experiments show a short lifetime of the population inversion allowing a generation of short (a few nanosecond) amplified light pulses in the Si/SiO2 lattice. The estimate for optical gain in the present samples is 6 cm−1 at 720 nm.

Graphene grown by high temperature molecular beam epitaxy on hexagonal boron nitride (hBN) forms continuous domains with dimensions of order 20 μm, and exhibits moiré patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moiré patterns are observed and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moiré periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moiré period reveal split and shifted G and 2D peaks confirming the presence of strain. Our work demonstrates a new approach to the growth of epitaxial graphene and a means of generating and modifying strain in graphene.

We report laser manipulations with stress at the nanoscale level. The continuous-wave Ar+ laser radiation melts Si nanocrystals in a free-standing Si∕SiO2 superlattice. Silicon crystallization from the liquid phase leads to a compressive stress, which can be accurately tuned in the 3GPa range using laser annealing below the Si melting temperature and then recovered by laser annealing above the melting temperature. This allows investigations of various phenomena as a function of stress and makes a case of Si-nanocrystal memory with very long retention time, which can be written, erased, and read by optical means.