Structural and Photoconductivity Properties of Tellurium/PMMA Films
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Owing to the very brittle nature of tellurium powder, nanoscopic grains with an average size of 4.8 ± 0.8 nm were produced by dry vibration milling technique using a mixer/mill apparatus. A novel material was obtained by binding the nanosized tellurium grains with poly(methyl methacrylate) (PMMA) polymer. The morphology, elemental composition, and structural and optical properties of Te/PMMA films were investigated. The prepared material was composed of hexagonal tellurium and α-phase of tellurium oxide. The electrical properties of the films were studied, for different electrode contact configurations, in dark condition and under white light illumination varying the optical power density from 2 to 170 mW/cm2 and turning the light on and off cyclically. Data analysis shows that the photoconductivity of the film with sandwich contact configuration is a linear function of the light power density and increases more than 2 orders of magnitude as compared to the photoresponse of the film with coplanar contact configuration.
KeywordsTellurium Vibration milling Poly(methyl methacrylate) Photoconductivity
Elemental tellurium is a p-type semiconductor that can be exploited for many technological applications in metallurgy, photovoltaics, photonics, electronics, and medicine . It has been used in the form of thin films or powder to fabricate gas sensors [2, 3], antiseptic materials , photoconductors [5, 6, 7], thermoelectric devices [8, 9, 10, 11], etc. Usually, chemical and electrochemical methods (“bottom-up” approaches), such as chemical vapor deposition  and solvothermal synthesis , are utilized to produce tellurium-based materials. In particular, one-dimensional (1D) tellurium nanostructures such as wires, rods, tubes, and belts have been synthesized. For example, Rao et al.  reported controlled synthesis of Te nanorods, nanowires, nanobelts, and related structures by the disproportionation of NaHTe in different solvents, Xia et al. [15, 16] prepared uniform Te nanowires and nanotubes through the reduction of H6TeO6 by N2H4H2O or ethylene glycol in refluxing process, and Qian’s group produced a series of 1D Te nanostructures including nanowires, nanobelts, and nanotubes via hydrothermal synthesis [17, 18, 19, 20]. Recently, Zhu et al.  presented an ultrasonic-assisted solution-phase approach for the fabrication of tellurium bundles of nanowhiskers, Sen et al.  synthesized Te nanostructures by physical vapor deposition, and Vasileiadis et al. demonstrated that a controlled fabrication of Te nanotubes can be carried out by irradiating bulk elemental Te with continuous wave lasers emitting in visible range for short exposure time .
In this paper, results on a top-down approach , based on dry vibration milling technology, to reduce the size of a brittle material such as tellurium and produce nanoscopic phases in a simple, effective, and inexpensive way, are reported. Indeed, nanostructures in the form of fine tellurium powder composed of grains with average size of a few nanometers were produced in air, without any temperature control and chemical reactions.
Furthermore, a novel functional material was obtained as monolithic film by binding the tellurium nanopowders with poly(methyl methacrylate) (PMMA), an amorphous thermoplastic polymer widely used in spectroscopic and optoelectronic applications.
Further advantages of this preparation method are the following: (i) the slight toxicity of tellurium is reduced by embedding it in the form of powder into a polymeric matrix thus overcoming the limits for an industrial use, (ii) the tellurium in a hyperfine form allows to achieve high homogeneous tellurium-polymer composites suitable for optical and flexible electronic applications, and (iii) the structural, optical, and electrical properties of the tellurium nanocomposites can be tuned by reducing the tellurium size at nanoscopic scale (tellurium quantum dots).
In order to obtain good electrical transport properties of this tellurium-based material, structures with a filling factor higher than 30 % by weight for nanoscopic tellurium  were prepared. The morphology, elemental composition, structural and optical properties of tellurium/PMMA films were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and UV-vis-NIR spectroscopies. The electrical properties were investigated in dark condition and under white light illumination with coplanar and sandwich electrode configurations. The time-dependent photocurrent responses were measured turning the light on and off cyclically at different optical power densities of the light.
Pure tellurium powder (99.8 % by weight, −200 mesh, Aldrich) was placed inside a steel grinding jar with two steel grinding balls. The tellurium grains were dry milled in air at a frequency of 25 Hz, for 7 h using a Mixer Mill apparatus (Retsch, MM-200). The grinding jar performs oscillations in a horizontal position, and the balls impact with high energy on the material thus pulverizing it. The movement of the grinding jar combined with the movement of the balls results in the intensive frictional action on the Te powder. At a frequency of 25 Hz, thousands of impacts per minute are achieved, resulting in a high degree of tellurium pulverization in a very short time.
The obtained powders were converted into monolithic samples by using a little amount of poly(methyl methacrylate) (M w = 996,000 g mol−1) as binder to fabricate Te/PMMA films of large area (ca. 20 cm2). In more detail, PMMA was dissolved in acetone at room temperature, then the powder was added and accurately dispersed by using a sonication bath, and finally, the liquid systems were spin-coated (60 min at 200 rpm.) on a silicon plate as substrate. The sample composition was 11 % by weight in polymer in order to reach a compromise between the minimum amount of polymer to bind the tellurium grains and the maximum concentration of tellurium to obtain suitable electrical properties for device applications. The thickness of the prepared Te/PMMA film, measured by a Millitron electrical length measuring instrument, was about 80 μm. The large-area film was cut into several specimens for each characterization.
The morphology of the tellurium powders and Te/PMMA films was investigated by SEM measurements performed by a FEI QUANTA 200 FEG apparatus equipped with an EDS microanalyzer (Inca Oxford 250), while nanoscopic grain size was determined by TEM measurements carried out by a FEI Tecnai G2 Spirit twin apparatus. In this case, the milled powder was dispersed into an amorphous polymeric matrix (polystyrene, Aldrich), using chloroform as solvent (solvent-mediated method) and depositing such tellurium/polystyrene system on the TEM copper grids. The structural characterizations of the powder samples and Te/PMMA films were carried out by X-ray diffraction and Fourier transform infrared spectroscopy using a PANalytical-X’Pert Pro diffractometer and Nicolet Nexus spectrophotometer, respectively.
The electrical properties were studied at room temperature in a coplanar configuration by depositing Ag paint contacts 4 mm long spaced by 1 mm and in a sandwich configuration covering the top and the bottom surfaces of the film with 3 and 10 mm2 of Ag contacts, respectively. The electrical measurements of the samples were performed in both contact configurations by a Keithley 6485 picoammeter and a Tektronics PS 280 DC power supply. Time-dependent current of the Te/PMMA films was measured switching on and off the white light illumination of an ELC 250 W lamp of General Electric. Optical power density of the light flux was varied from 2 to 170 mW/cm2 by means of neutral density filters and measured by a Laser Precision Rk-5720 power radiometer.
Results and Discussion
Structural and Morphological Analysis
Te/PMMA film of large area was obtained by binding the nanoscopic Te grains of the milled powder with PMMA.
Optical and Photoconductivity Properties
The very different dark conductivity values obtained for the two configurations can be attributed to the film morphology as shown by the SEM image in Fig. 4.
According to refs. [6, 23, 28], the origin of photoconduction in this material may be attributed to the coexistence of Te and TeO2 phases due to the partial oxidation of Te grains as demonstrated by XRD analysis.
It has been demonstrated that dry vibration milling is a suitable technology for producing tellurium nanoscopic powders. A novel material based on nanosized tellurium bound by means of a thermoplastic polymer such as PMMA was prepared.
The morphological characterizations of Te powders and Te/PMMA films were performed by SEM and TEM, and the films showed a quite porous nature due to the low amount of polymer present in the film. From XRD data analysis, it was obtained that the Te/PMMA material was composed of hexagonal Te and α-phase of tellurium oxide. The optical absorptance of the Te/PMMA sample was found varying in the 0.8–0.9 range in the UV-vis-NIR region. The time-dependent photoconductivity properties of Te/PMMA films were explored under white light illumination, turning the light on and off cyclically in coplanar and sandwich contact configurations. The photoresponse was studied as a function of the optical power density in the 2–170 mW/cm2 range. In the coplanar configuration, the rise and decay times of the photocurrent signal are on the order of hundreds of seconds, while in the sandwich configuration, the signal varies faster and the rise and decay times are just a few seconds. Finally, for the sandwich configuration, a linear correlation between the photocurrent and optical power density, and higher values of the photoresponse were found.
We gratefully acknowledge Dr. Carla Minarini of ENEA-Portici Research Centre for the Optical Measurements and Maria Cristina del Barone of LAMEST laboratory IPCB-CNR for SEM and TEM analysis.
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