Carotenuto et al. Nanoscale Research Letters (2015) 10:313 DOI 10.1186/s11671-015-1007-z

NANO EXPRESS Open Access

Structural and Photoconductivity Propertiesof Tellurium/PMMA Films

Gianfranco Carotenuto1, Mariano Palomba1*, Sergio De Nicola2*, Giuseppina Ambrosone2,3 and Ubaldo Coscia3,4

Abstract

Owing to the very brittle nature of tellurium powder, nanoscopic grains with an average size of 4.8 ± 0.8 nm wereproduced by dry vibration milling technique using a mixer/mill apparatus. A novel material was obtained by binding thenanosized 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 ofhexagonal tellurium and α-phase of tellurium oxide. The electrical properties of the films were studied, for differentelectrode contact configurations, in dark condition and under white light illumination varying the optical power densityfrom 2 to 170 mW/cm2 and turning the light on and off cyclically. Data analysis shows that the photoconductivity of thefilm with sandwich contact configuration is a linear function of the light power density and increases more than 2 ordersof magnitude as compared to the photoresponse of the film with coplanar contact configuration.

Keywords: Tellurium; Vibration milling; Poly(methyl methacrylate); Photoconductivity

BackgroundElemental tellurium is a p-type semiconductor that canbe exploited for many technological applications in me-tallurgy, photovoltaics, photonics, electronics, and medi-cine [1]. It has been used in the form of thin films orpowder to fabricate gas sensors [2, 3], antiseptic mate-rials [4], photoconductors [5–7], thermoelectric devices[8–11], etc. Usually, chemical and electrochemicalmethods (“bottom-up” approaches), such as chemicalvapor deposition [12] and solvothermal synthesis [13],are utilized to produce tellurium-based materials. In par-ticular, one-dimensional (1D) tellurium nanostructuressuch as wires, rods, tubes, and belts have been synthe-sized. For example, Rao et al. [14] reported controlledsynthesis of Te nanorods, nanowires, nanobelts, and re-lated structures by the disproportionation of NaHTe indifferent solvents, Xia et al. [15, 16] prepared uniformTe nanowires and nanotubes through the reduction ofH6TeO6 by N2H4H2O or ethylene glycol in refluxingprocess, and Qian’s group produced a series of 1D Tenanostructures including nanowires, nanobelts, andnanotubes via hydrothermal synthesis [17–20]. Recently,

* Correspondence: mariano.palomba@cnr.it; sergio.denicola@spin.cnr.it1Institute for Polymers, Composites and Biomaterials, National ResearchCouncil, Piazzale E. Fermi 1, 80055 Portici, Naples, Italy2SPIN Institute, National Research Council, via Cintia, 80126 Naples, ItalyFull list of author information is available at the end of the article

© 2015 Carotenuto et al. Open Access This aLicense (http://creativecommons.org/licenseany medium, provided the original work is

Zhu et al. [21] presented an ultrasonic-assisted solution-phase approach for the fabrication of tellurium bundlesof nanowhiskers, Sen et al. [22] synthesized Te nano-structures by physical vapor deposition, and Vasileiadiset al. demonstrated that a controlled fabrication of Tenanotubes can be carried out by irradiating bulk elemen-tal Te with continuous wave lasers emitting in visiblerange for short exposure time [23].In this paper, results on a top-down approach [24],

based on dry vibration milling technology, to reduce thesize of a brittle material such as tellurium and producenanoscopic phases in a simple, effective, and inexpensiveway, are reported. Indeed, nanostructures in the form offine tellurium powder composed of grains with averagesize of a few nanometers were produced in air, withoutany temperature control and chemical reactions.Furthermore, a novel functional material was obtained

as monolithic film by binding the tellurium nanopowderswith poly(methyl methacrylate) (PMMA), an amorphousthermoplastic polymer widely used in spectroscopic andoptoelectronic applications.Further advantages of this preparation method are the

following: (i) the slight toxicity of tellurium is reducedby embedding it in the form of powder into a polymericmatrix thus overcoming the limits for an industrial use,(ii) the tellurium in a hyperfine form allows to achieve high

rticle is distributed under the terms of the Creative Commons Attributions/by/4.0), which permits unrestricted use, distribution, and reproduction inproperly credited.

Carotenuto et al. Nanoscale Research Letters (2015) 10:313 Page 2 of 8

homogeneous tellurium-polymer composites suitable foroptical and flexible electronic applications, and (iii) thestructural, optical, and electrical properties of the telluriumnanocomposites can be tuned by reducing the telluriumsize at nanoscopic scale (tellurium quantum dots).In order to obtain good electrical transport properties

of this tellurium-based material, structures with a fillingfactor higher than 30 % by weight for nanoscopic tellur-ium [25] were prepared. The morphology, elemental com-position, structural and optical properties of tellurium/PMMA films were analyzed by scanning electron micros-copy (SEM), energy-dispersive spectroscopy (EDS), trans-mission electron microscopy (TEM), X-ray diffraction(XRD), Fourier transform infrared (FT-IR), and UV-vis-NIRspectroscopies. The electrical properties were investigatedin dark condition and under white light illumination withcoplanar and sandwich electrode configurations. The time-dependent photocurrent responses were measured turningthe light on and off cyclically at different optical powerdensities of the light.

MethodsPure tellurium powder (99.8 % by weight, −200 mesh,Aldrich) was placed inside a steel grinding jar with twosteel grinding balls. The tellurium grains were dry milledin air at a frequency of 25 Hz, for 7 h using a Mixer Millapparatus (Retsch, MM-200). The grinding jar performsoscillations in a horizontal position, and the balls impactwith high energy on the material thus pulverizing it. Themovement of the grinding jar combined with the move-ment of the balls results in the intensive frictional actionon the Te powder. At a frequency of 25 Hz, thousandsof impacts per minute are achieved, resulting in a highdegree of tellurium pulverization in a very short time.The obtained powders were converted into monolithic

samples by using a little amount of poly(methyl meth-acrylate) (Mw = 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 byusing a sonication bath, and finally, the liquid systemswere spin-coated (60 min at 200 rpm.) on a silicon plateas substrate. The sample composition was 11 % by weightin polymer in order to reach a compromise between theminimum amount of polymer to bind the tellurium grainsand the maximum concentration of tellurium to obtainsuitable electrical properties for device applications. Thethickness of the prepared Te/PMMA film, measured by aMillitron electrical length measuring instrument, wasabout 80 μm. The large-area film was cut into severalspecimens for each characterization.The morphology of the tellurium powders and Te/

PMMA films was investigated by SEM measurements per-formed by a FEI QUANTA 200 FEG apparatus equipped

with an EDS microanalyzer (Inca Oxford 250), whilenanoscopic grain size was determined by TEM measure-ments carried out by a FEI Tecnai G2 Spirit twin appar-atus. In this case, the milled powder was dispersed into anamorphous polymeric matrix (polystyrene, Aldrich), usingchloroform as solvent (solvent-mediated method) and de-positing such tellurium/polystyrene system on the TEMcopper grids. The structural characterizations of the pow-der samples and Te/PMMA films were carried out byX-ray diffraction and Fourier transform infrared spectros-copy using a PANalytical-X’Pert Pro diffractometer andNicolet Nexus spectrophotometer, respectively.The electrical properties were studied at room

temperature in a coplanar configuration by depositing Agpaint contacts 4 mm long spaced by 1 mm and in a sand-wich configuration covering the top and the bottom sur-faces of the film with 3 and 10 mm2 of Ag contacts,respectively. The electrical measurements of the sam-ples were performed in both contact configurationsby a Keithley 6485 picoammeter and a Tektronics PS280 DC power supply. Time-dependent current of theTe/PMMA films was measured switching on and offthe white light illumination of an ELC 250 W lampof General Electric. Optical power density of the lightflux was varied from 2 to 170 mW/cm2 by means ofneutral density filters and measured by a Laser Preci-sion Rk-5720 power radiometer.

Results and DiscussionStructural and Morphological AnalysisSEM and TEM measurements on Te powder sampleswere carried out to verify the ability of dry vibrationmilling technique to produce nanoscopic Te powder forthe fabrication of Te/PMMA films. The “as received” tel-lurium powder was made of quite monodispersed pseu-dospherical grains with an average size of ca. 30 μm asvisible in the SEM micrograph given in Fig. 1a, whereasthe milled powder was characterized by a polymodalparticle size distribution as displayed in the SEM micro-graph in Fig. 1b. Most part of the powder was made ofnanoscopic tellurium grains, as can be seen from theTEM image in Fig. 1c. The grain size distribution isshown in the histograms in Fig. 1d, and the averagegrain diameter was estimated to be 4.8 ± 0.8 nm.The composition of the milled powder was determined

by EDS investigation (see Fig. 2). Data analysis indicatesthe presence of oxygen, due to the dry-milling processperformed in air that can lead to a partial oxidation ofthe tellurium grain surface, and traces of iron, which isalready present in the starting coarse tellurium powder.Some information on the structural properties of the

milled powder was obtained by infrared spectroscopy. In-deed, the FT-IR spectrum of the milled tellurium sampleshown in Fig. 3 includes the typical absorption bands of

Fig. 1 SEM micrographs of the “as received” tellurium powder (a) and the tellurium powder after milling (b). TEM-micrograph of the achievednanoscopic tellurium grains embedded into an amorphous polystyrene matrix (c). Tellurium grain size distribution (d)

Carotenuto et al. Nanoscale Research Letters (2015) 10:313 Page 3 of 8

tellurium oxide centered at wavenumbers of 773 and667 cm−1. According to the literature [26], these two reso-nances correspond to the symmetrical equatorial andasymmetrical axial stretching frequencies of the Te–Obonds, respectively. Furthermore, a quite broad and inten-sive absorption band due to the OH stretching vibrationalmode, located at a wavenumber of 3436 cm−1, can bedetected revealing that the oxide phase was partiallyhydrated.Te/PMMA film of large area was obtained by binding

the nanoscopic Te grains of the milled powder withPMMA.

Fig. 2 EDS spectrum of a milled tellurium powder sample

The morphology of the film was investigated bySEM. The micrograph, shown in Fig. 4, reveals thatthe material was quite porous, due to the very highinorganic phase content. In particular, both macro-and micro-porosities can be clearly observed as blackareas in the image.The structural properties of the Te/PMMA material

were studied by XRD measurements. In Fig. 5a, the XRDdiffractogram shows a prominent peak at 27.32° belongingto the diffraction pattern of the hexagonal tellurium(JCPDS card 36-1452) and two other less intensive diffrac-tion peaks at 25.99° and 29.79° due to the α-phase of

Fig. 3 FT-IR spectrum of a milled tellurium powder sample

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tellurium oxide (α-TeO2, JCPDS card 78-1713). Both Teand α-TeO2 phases were already present in the “as re-ceived” Te powder, as shown in the XRD spectrum ofFig. 5b; however, in the case of the Te/PMMA film, thepeaks are broadened because the sample was composed ofnanoscopic Te grains, achieved at the end of the millingprocess. Furthermore, according to the integrated area ofthe diffraction peaks, the composition of the film was53 % Te and 47 % α-TeO2, while for the “as received”powder it was 80.8 % Te and 19.2 % α-TeO2. The highervalue of the α-TeO2 phase percentage in the film can beascribed to an oxidation process of the Te grains occur-ring during the milling stage.

Fig. 4 SEM micrograph of the Te/PMMA film

Optical and Photoconductivity PropertiesThe optical properties of the Te/PMMA film were inves-tigated by means of transmittance (T) and reflectance(R) spectroscopy in the UV-vis-NIR range. The R/Tspectra of the film are shown in Fig. 6. The transmit-tance increases in the 200–350 nm range, and it is quiteconstant in the Vis and NIR ranges. The reflectancerapidly decreases in the UV region, slightly decreasesin the Vis region, and is quite constant in the NIRregion. Thus, from quantitative analysis, the sampleabsorptance varies in the 0.8–0.9 range in all theUV-vis-NIR regions.The electrical characterization of the Te/PMMA sam-

ples was performed both in coplanar and sandwich con-figurations. In the dark condition, the I–V characteristicin the coplanar configuration is nonlinear (see Fig. 7a)and the behavior of the absolute value of the current asa function of the applied voltage V is exponential asshown in the semilogarithmic plot of the inset in Fig. 7a.In the sandwich configuration, the I–V characteristic,displayed in Fig. 7b, is quite linear indicating that thecontacts are ohmic.The very different dark conductivity values obtained

for the two configurations can be attributed to the filmmorphology as shown by the SEM image in Fig. 4.In order to investigate the photoconductivity prop-

erties, the samples were illuminated by white light ofdifferent power density values. The current under il-lumination, Ilight, was measured in coplanar configur-ation, by applying a bias voltage of 200 V to reach a

Fig. 5 XRD diffractogram of the Te/PMMA film (a) and “as received” tellurium powder (b)

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good signal-to-noise ratio avoiding high power dissi-pation. The illumination exposure time was held fixedat 440 s and immediately after the light was switchedoff to obtain light-dark cycles and evaluate the photo-current, Iph, as Iph = Ilight − Idark, where Idark was thedark current measured before turning on the light.The evolution of Iph as a function of time for light-dark cycles performed by light power densities of 100,120, and 170 mW/cm2 is shown in Fig. 8. The photo-current signal slowly increases under illumination,and the rise time varies in the 100–300 s range. Uponturning off the light, the signal decreases slowly andthe decay time varies in the same range. The inset inFig. 8 shows the maximum value of the photocurrentfor each cycle, Iphmax, as a function of power densityF in the 10–170 mW/cm2 range.In order to compare the photoconductivity measure-

ments with the literature data concerning the coplanarconfiguration, the Imax/Idark ratio versus the light powerdensity F is plotted in Fig. 9, where Imax = Iphmax + Idark isthe maximum value of the current before turning off thelight. The Imax/Idark ratio depends quasilinearly on F, andit is worth noting that Imax/Idark = 2.8 at F = 100 mW/cm2;thus, the Te/PMMA sample shows a good photosensitivity

Fig. 6 Transmittance, T, and reflectance, R, spectra of the Te/PMMAfilm in the UV-vis-NIR region

as in the case of films composed of tellurium nanorodsimmersed in polydimethylsiloxane [27].The photoresponse of the Te/PMMA sample in the

sandwich configuration for different light-dark cyclesis shown in Fig. 10. The measurements were carriedout by applying a bias voltage of 12 μV to the samplevarying the light power density from 2 to 170 mW/cm2. The duration of the illumination exposure wasfixed at 60 s for each cycle. In this case, the rise time

Fig. 7 I–V characteristics of the Te/PMMA films in coplanarconfiguration (a) and sandwich configuration (b). In the inset,the absolute value of Idark, abs (I), as a function of the appliedvoltage V is plotted in semi-log scale

Fig. 8 Time-dependent photocurrent, Iph, measurements for differentlight-dark cycles at various light power densities (100, 120, and 170 mW/cm2) for coplanar configuration. In the inset, the maximum photocurrentobtained for each cycle, Iphmax, versus light power density, F, is displayed

Fig. 10 Time-dependent photocurrent Iph measurements for differentlight-dark cycles with light power density F ranging from 15 to 170 mW/cm2 for sandwich configuration. In the inset, the maximum photocurrentobtained for each cycle, Iphmax, is plotted as a function of light powerdensity F

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of the signal was about 4 s. When the light is re-moved, the current quickly decreases with a decaytime of about 2 s. The inset in Fig. 10 shows that Iph-max depends linearly on F.According to refs. [6, 23, 28], the origin of photocon-

duction in this material may be attributed to the coexist-ence of Te and TeO2 phases due to the partial oxidationof Te grains as demonstrated by XRD analysis.In order to compare the photoresponse of the samples

for the two contact configurations, the photocurrent Iphmax

is plotted as a function of F in Fig. 11 in bi-logarithmicscale. Clearly, in the case of the sandwich configuration,the photocurrent obtained applying only tens of microvolts

Fig. 9 Imax/Idark ratio (where Imax is the maximum current obtainedfor each light-dark cycle and Idark is the current in dark condition)versus light power density F for Te/PMMA film incoplanar configuration

of bias voltage is more than 2 orders of magnitude greaterthan the one measured in the coplanar configuration byapplying 200 V.

ConclusionsIt has been demonstrated that dry vibration milling is asuitable technology for producing tellurium nanoscopicpowders. A novel material based on nanosized telluriumbound by means of a thermoplastic polymer such asPMMA 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

Fig. 11 The maximum photocurrent obtained for each light-darkcycle, Iphmax, versus light power density, F, for sandwich andcoplanar configurations

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low amount of polymer present in the film. From XRDdata analysis, it was obtained that the Te/PMMA mater-ial was composed of hexagonal Te and α-phase of tellur-ium oxide. The optical absorptance of the Te/PMMAsample was found varying in the 0.8–0.9 range in theUV-vis-NIR region. The time-dependent photoconduct-ivity properties of Te/PMMA films were explored underwhite light illumination, turning the light on and off cyc-lically in coplanar and sandwich contact configurations.The photoresponse was studied as a function of the op-tical power density in the 2–170 mW/cm2 range. In thecoplanar configuration, the rise and decay times ofthe photocurrent signal are on the order of hundredsof seconds, while in the sandwich configuration, thesignal varies faster and the rise and decay times arejust a few seconds. Finally, for the sandwich configur-ation, a linear correlation between the photocurrentand optical power density, and higher values of thephotoresponse were found.

Competing interestsThe authors declare that they have no competing interests.

Authors’ contributionsGC conceived of the experimental design and co-wrote the paper. SDNco-wrote the paper and performed the experimental measurements. UCand GA developed and performed the photoelectrical characterizationsand co-wrote the paper. MP participated in the design of the experimentand developed the sample preparation. All authors read and approved thefinal manuscript.

Authors’ informationGC is a senior researcher at the Institute for Polymers, Composites andBiomaterials, Italian National Research Council. His present researchinterests are in the field of advanced functional materials based onpolymer-embedded inorganic nanostructures. In particular, his activityconcerns the development of new chemical routes for the controlledsynthesis of metal and semiconductor clusters in polymeric matrices, thefabrication of devices based on properties of nanoscopic objects(luminescence of quantum dots, tunable surface plasmon absorption ofnanosized noble metal alloys, etc.), and the investigation of mechanismsinvolved in atomic and molecular cluster formation in polymeric media(nucleation, growth, aggregation, etc.) by optical and luminescencespectroscopy. He has authored 150 research articles published in internationaljournals, ten patents, and many conference papers. He is the editor of twoWiley books devoted to metal-polymer nanocomposites and is a member ofthe editorial board of different scientific journals.SDN received his BS degree in physics from the University of Naples“Federico II”, Italy, in 1982. From 1983 to 1987, he was a system analyst atElettronica (Rome) and Alenia (Naples). Since 1988, he has been a staffresearcher at the Institute of Cybernetics “E. Caianiello” of the NationalResearch Council. Currently, he is a senior researcher at the SPIN Institute(Institute for Superconductors, Oxides and Other Innovative Materials andDevices), National Research Council (CNR). He has been a scientificcoordinator of the research project ‘Imaging Techniques for Studying andAnalyzing Microstructured Materials’ of the Department of Physics Sciencesand Matter Technologies (DSFTM) of the National Research Council. He hasbeen a coordinator of the research unit based at the Institute of Cyberneticsin the framework of the Italian National Research FIRB program: PhotonicMicrodevices in Lithium Niobate. He has contributed to about 300 technicalpapers in peer-reviewed international journals, book chapters, and conferenceproceedings. He has served in program committees of several internationalconferences and has been a referee for various journals in the field of opticsand theoretical physics. His research interests include the development ofquantum methodologies to the description of coherent phenomena in many

body systems, quantum tomography, theoretical modeling for studying dynam-ical effects in mesoscopic systems and nanostructured polymeric materials, elec-tronic coherent transport in nonconventional superconductors and graphene,and interaction of optical and electron beams in nonlinear media and plasma.UC is a researcher at the Physics Department of University of Naples “FedericoII”. He is a scientific coordinator of the “AmorphousSemiconductors” Laboratory,and the main research activity is based on the deposition by PECVD of micro/nano crystalline silicon, silicon-carbon alloys films and their optical, structural,and electrical characterizations for applications in photodetectors and solar cells.The results of his scientific activity are reported in papers published ininternational journals, conference proceedings, and scientific books.GA is a researcher at the Naples University “Federico II”, and her scientificactivity is carried out in the “Amorphous semiconductor” laboratory of thePhysics Department. In the last years, she studied the optical, electrical,structural, and radiative properties of silicon-based thin films deposited byPECVD for optoelectronic applications. She collaborates with national/international research groups and participated in relevant projects. Herarticles were published in international journals, scientific books, andconference proceedings.MP, PhD in “Materials and Structures Engineering,” degree in ChemicalEngineering, is currently a researcher at the Institute for Polymers,Composites and Biomaterials, National Research Council (IPCB-CNR) ofNaples. His current scientific interests are related to the thermo-chemicalsynthesis of metal-polymer nanocomposites and their morphological andstructural characterizations, realized by electron microscopy (SEM, TEM) andX-ray powder diffraction (XRD) and optical spectroscopy techniques(UV-visible absorption and emission spectroscopy) to analyze the relationamong chemical-physical properties and the nature, size, and shape of thesenanomaterials. Furthermore, he studies the mechanical synthesis of brittlemetalloids at nanoscopic scale and conductive and photoconductiveproperties of this systems embedded in polymers matrices, and the mechanicaldeposition of graphene on polymers substrates and its functionalization withchemical groups. He has several dozens of scientific papers on internationaljournals and several participations at international conferences.

AcknowledgementsWe gratefully acknowledge Dr. Carla Minarini of ENEA-Portici ResearchCentre for the Optical Measurements and Maria Cristina del Barone ofLAMEST laboratory IPCB-CNR for SEM and TEM analysis.

Author details1Institute for Polymers, Composites and Biomaterials, National ResearchCouncil, Piazzale E. Fermi 1, 80055 Portici, Naples, Italy. 2SPIN Institute,National Research Council, via Cintia, 80126 Naples, Italy. 3Department ofPhysics, University of Naples ‘Federico II’, via Cintia, 80126 Naples, Italy.4CNISM, Naples Unit, via Cintia, 80126 Naples, Italy.

Received: 24 April 2015 Accepted: 7 July 2015

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