Novel Solid Nano Diamond/Pyrocarbon Semiconductor Materials

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Novel Solid Nano Diamond/Pyrocarbon Semiconductor Materials Sergey K. Gordeev*, Peter I. Belobrov†, Nikolay I. Kiselev†, Eleonora A. Petrakovskaya†, Thommy C. Ekstrom‡ *Central Research Institute for Materials, St. Petersburg, RUSSIA † Kirensky Institute of Physics & Institute of Biophysics SB RAS, Krasnoyarsk, RUSSIA ‡ Skeleton Technologies Group, Stockholm, SWEDEN ABSTRACT Porous solids made of nano diamonds cemented by pyrocarbon show a semiconductor behavior where the properties are controlled by the factor γ = mass ratio of sp2/sp3 phases. The volume electrical conductivity σ (T) was measured in the temperature range 77 - 290 K as a function of γ. By controlling the ratio γ from 0 to 50 %, the electrical conductivity will change by 12 orders of magnitude. The semiconductor activation energy depends on γ-ratio and has values between 0.03 - 0.3 eV. The variable-range hopping conduction was checked by measuring σ against T and to look for a T1/4 dependence. A positive thermoelectric power showed that NDC was p-semiconductor with a narrow band gap. The paramagnetic nanodiamonds, g = 2.0027(1) and ∆H = 0.86(2) mT, will still have this property in the NDC material and both the g-value and line width are not dependent on γ-ratio and temperature. The concentration of unpaired electrons is 4*1019 spin/g that equals to one dangling bond per nanodiamond particle. Absences of magnetoresistance and Hall potential were observed for all NDC materials. This was interpreted as resulting from a Tamm - Lifshitz - Pekar electronic conduction mechanism within onedimensional band of surface states.

INTRODUCTION Diamond nano crystals are synthesized by an explosion method [1, 2]. It is well known that single and micro crystals of diamond are not thermodynamic stable in ambient conditions, whereas nanodiamonds surprisingly are more stable than graphite [3, 4]. This was used for making novel porous carbon nano composites [5, 6]. These nano diamond/pyrocarbon composites (NDC), made from nanodiamond bound by pyrocarbon, can be prepared as solid bulk materials and not only as films. Tamm introduced surface electronic states into physics [7]. Developing this idea Lifshitz and Pekar have constructed a qualitative physical pattern by introducing one-dimensional bands of electron surface states, relating them with charge and spin carrier transfer on the edges of small micro crystals [8]. At the time the idea was proposed there were no adequate materials to verify the phenomenon. Our aim was to characterize the new class of porous NDC materials prepared as a series with controlled compositions. Experimental data are presented below and discussed. The unusual semiconductor properties observed show that NDC represents a new family of modern carbon materials predicted earlier in theory only [8]. The physical properties are defined by the considerable contribution of quantum size effects. The various NDC materials are interesting


both as scientific model materials of porous bodies as well as in applications requiring components with s