General, as pre-edge features104. These properties were related

General, measuring the optoelectronic characteristics of nanodiamond
and diamondoids has turned out to be challenging and has give several
controversial outcomes. In 1999, the results of x-ray absorption near edge
structure investigations of diamond films produced by chemical vapour deposition (CVD) were employed to deduce
the advancement of the nanoparticle gap with size102. This release a
resolution of quantum confinement effects up to 27 nm, a diameter surprisingly greater than
Si or Ge nanoparticles, where quantum confinement effects disappear above 5–7
nm. In contradiction, recent near-edge absorption fine structure investigations
of diamondoids prepared by hot filament CVD and high-explosive detonation waves
revealed that quantum confinement effects disappear in particles greater than 4
nm. Individually, it was presented that there is no variation in valence and
conduction band i.e maximum and minimum in detonation nanodiamonds, in contrast
with bulk diamonds103. This can be noticed that
quantum confinement does not involve in the electronic structure of the
measured particles i.e in the size range of 4 nm. The bulk and nanodiamonds shows the similar X-ray emission
and absorption spectra, with an exciton broadening (289.3 eV) and a shallower
secondary minimum (302 eV) as pre-edge features104. These properties were related
to individual surface reconstructions such as in bucky diamonds.

        The optical properties of UDD layers have been
studied by optical probes and by XPS. The band gap was measured to be smaller
than the diamond i.e 3.5 eV, and many energy levels were present in the
nanodiamond band gap, contributing to a broad luminescence band (380–520 nm)105. The optical absorption of the
material was related to the threefold coordinated atoms on the surface. Authors
studied the size dependency of the optical gap of diamondoids using both
time-independent and time-dependent DFT predictions
and observe that quantum confinement effects will no longer in nanoparticles of
size larger than 1 nm. They also concluded that the gaps of diamondoids with
sizes between 1 and 1.5 nm are below the gap of bulk diamond. This is
strikingly different from the behaviour
of H-terminated Si and Ge nanoparticles, for which the gaps are consistently
above the bulk band gap. In contrast, according to Density Functional Theory
(DFT) calculations done by authors for the same particles, it is predicted that
optical gaps is 2 eV above the gap of bulk diamond for the particles ranging in
size from 0.5 to 2 nm106. Highly accurate quantum Monte
Carlo (QMC) calculations resolved this controversy, showing the disappearance
of quantum confinement at about 1 nm. It was also pointed out that the results may be influenced by basis set superposition
errors, and these are considered to be responsible for the discrepancy between
results achieved with localized and plane wave basis sets107. In extension, Drummond et al.
predicted that diamondoids show negative electron affinity103.

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      The band gap depends on the size of
nanoparticles. Two important class of nanoparticles have been investigated: i)
diamondoids constructed from adamantine cages: adamantane, C10H16,
diamantane, C14H20, and pentamantane,
C26H32; (ii) H-terminated, spherical, diamond structure
nanoparticles: C29H36, C66H64, and
C87H76108. Because diamondoids can be
extracted in large quantities from petroleum and are highly purified by using
high-pressure liquid chromatography, one can expect that actual experimental
samples contain largely of the high-symmetry structures studied theoretically.
This is not the case for Si and Ge nanoparticles, where limitations in current
synthesis techniques prevent the routine production of high symmetry
nanoparticles109. 

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