- Email: [email protected]
Creation and study of 2D and 3D carbon nanographs
Physica E 44 (2012) 976–980
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Creation and study of 2D and 3D carbon nanographs A.O. Golubok a,b, I.S. Mukhin c,n, I.U. Popov a, I.S. Lobanov a a
Saint-Petersburg State University of Information Technologies, Mechanics and Optics, Kronverksky Pr., 49, St. Petersburg 197101, Russian Federation Institute for Analytical Instrumentation of the Russian Academy of Sciences, Rizhsky Pr., 26, St. Petersburg 190103, Russian Federation Institution of the Russian Academy of Sciences, Saint-Petersburg Academic University—Nanotechnology Research and Education Centre RAS, Physics and Technologies Nanostructures, Khlopina St., 8/3, St. Petersburg 194021, Russian Federation b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2010 Accepted 22 October 2010 Available online 21 November 2010
The technology of nanowhiskers growth in the scanning electron microscope vacuum chamber have been applied for the creation of 2D and 3D carbon wireframe nanosystems (CWNS). The probability of electron transition has been calculated within simple quantum graph (quantum network) model. For the purpose of research of CWNS electron state density investigation it is proposed to grow CWNS on the tungsten tip apex and to measure current–voltage characteristics I(V) of Au plate – vacuum gap – CWNS W tip tunneling junction in the ultra high vacuum scanning tunneling microscope. Experiments show that proposed methodic approach gives adequate I(V) dependencies for metallic and semiconductor nanowhiskers at the room temperature. For detection of the peculiarities at I(V) tunneling junction characteristics caused by quantum graphs it is required to carry out measurements at low temperature conditions. & 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Technology of creation of carbon wireframe nanosystems
Creation, study and application of different solid state nanosystems based on 2D, 1D and 0D structural elements with quantum scale properties constitute an important direction of modern nanoscale science and technology. Semiconductor lasers based on quantum dots or quantum wire ensembles are an impressive example [1]. From the viewpoint of both the fundamental theory and its application it is interesting to create and study 2D and 3D quantum graph nanosystems containing 1D nanostructures as their edges. These systems are called wireframe nanosystems. According to the formal point of view it is a solution of the ¨ Schrodinger equation within 1D systems which are curved by the so-called hybrid manifold modulation with various dimensions. It can be electron state density features at the quantum graph nodes. Generally speaking, this quantum graph structure overpatching for example angle leaves changing can be used for the electron state density management. Therefore, the quantum graphs’ growth technology might provide for the opportunities of the well-defined properties’ nanosystem construction, including nanoelectronically and optoelectronically nonlinear elements. Apart from that, wireframe nanosystems can be used as elements of the micro- and nanomechanical devices, such as acceleration gages, cantilevers , liquid metal cathodes, field emission cathodes and nanotweesers.
Our technology of creation of carbon wireframe nanosystems is based on a single nanowhisker growth technology. The known nanowhisker growth methods are based on various physical– chemical phenomena, such as the molecular beam epitaxy, metalorganic chemical vapor deposition, electrolysis, laser ablation, focused beam electron deposition, etc. The focused electron beam material deposition method is used. As opposed to Ref. [2], the nanowhisker geometry growth monitoring is used. The experimental setup is presented in Fig. 1(a). A whisker growth substrate is placed in the scanning electron microscope vacuum chamber. The substrate might be planar or have a tip form. A carbon-containing target was placed near the substrate for acceleration of the growth processes. The 10 keV and 50 pA electron beam is focused at a whisker growth place. The electron beam illuminates both the substrate and the target surface. A 20–50 nm nanobump appears on the substrate surface after the focused beam short exposure. The nanowhisker growth is defined by an electron beam spatial movement. The nanowhisker growth direction is perpendicular to the electron beam. A focused beam shift trajectory defines a nanowhisker growth form. In that design the focused electron beam is a spatial drawing nanopan. It is important to mention that a minimal growth whisker dimension is bigger than an electron beam spot dimension (about 2 nm). That is why the electron beam scans a small 15 15 nm2 area during the exposure. The nanowhisker growth form is controlled by a bigger area scanning with secondary electrons during growth processes. This approach provides for carbon whisker growth opportunities
n
Corresponding author. Tel.: + 79516610258. E-mail address: [email protected] (I.S. Mukhin).
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.10.013
A.O. Golubok et al. / Physica E 44 (2012) 976–980
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Fig. 1. (a) Whiskers and wireframe graph systems growth experimental setup: 1—the focused electron beam; 2—the substrate; 3—the carbon containinig target; 4—carbon ions; 5—grown whisker. (b) Simulation results of the whisker grown at the top of the tungsten tip.
and a whisker length is ten times larger than a typical lateral whisker size. It is important to mention that the whisker cross section does not have symmetric form with an extension along the beam direction. A simple whisker growth model is suggested in Ref. [3]. According to this model it is supposed that a positive carbon ion region is induced near the substrate surface by the focused electron 10–30 keV beam. The carbon ions interact with electron beam field. In this model there is a dedicated direction which is parallel to the electron beam axis. Carbon ions leave the substrate, oscillate near the beam axis and fill relatively small area near the electron beam (a surface growth size along the electron beam direction is bigger than the carbon ions localization area near the electron beam). This fact reflects asymmetric form of the whisker cross section. It is supposed that carbon ions neutralize at the substrate surface and absorb there under the secondary electrons impact. Model simulation shows that change of controlling parameters, for example, acceleration of voltage, electron beam current, beam diameter, distance between the target and the substrate, might impact whisker growth processes, change the whisker cross section and make more symmetrical form. Model simulation results are presented in Fig. 1(b). It should be observed that the whisker growth took place also in the vacuum chamber without the carbon target, but the growth rate in this condition is very insignificant. According to X-ray spectral microanalysis data, it was shown that a whisker material is carbon too. It is supposed that this fact is connected with carbon presence in vacuum chamber SEM residual atmosphere. Therefore, it is possible to create spatial carbon wireframe nanosystems (CWNS) by the focused electron beam shifting in the focal beam plane vis-a -vis the substrate and the substrate shifting with the SEM goniometer visa -vis the electron beam. Fig. 2 shows 2D wireframe nanosystems containing whiskers, those structures have been grown at the tungsten tip surface.
¨ 3. Schrodinger equation of 1D curvilinear systems Theoretical prediction of tunneling spectroscopy data through carbon whisker can be obtained using quantum graphs model [4]. The simple graph containing a loop formed by whiskers Ea and Eb attached to the tip Et and separated from the sample Es by the vacuum region Ev is considered. It is assumed that the electron motion in whiskers as well as in tip, vacuum and sample is quasi one-dimensional, therefore the configuration spaces for regions Ea, Eb and Ev is chosen to be finite segments of length la, lb and lv, respectively, and the configuration spaces for Es and Ev are halflines. The resulting quantum graph is shown in Fig. 3.
Electron scattering in similar systems was considered earlier (see Ref. [5] and references therein), however tunneling region and electric fields have been neglected up to now. The Hamiltonian of the considered system coincides with the ¨ convenient one-dimensional Schrodinger operator on wave functions vanishing outside one of segments Ee, eAI¼{a, b, v} hence the Hamiltonian can be obtained as the self-adjoint extension of the ¨ direct sum of Schrodinger operators on segments Ee defined on functions vanishing with derivatives at ends of segments onto functions on whole system satisfying to the Kirchhoff conditions (a continuous function is said to satisfy the Kirchhoff conditions, if the sum of external derivatives is equal to zero at every point). It is assumed that the scalar potential on whiskers is constant, and the potential on the vacuum region is approximated by a linear potential. The typical dependence of the transition probability from the tip to the sample on the applied energy is shown at Fig. 4 for the whiskers of 100 nm length, tunneling region of 6 nm width, potential barrier of 4 eV height under applied 0–8 V voltage, electron kinetic energy is equal to Fermi level energy. Fig. 4 shows that transition probability has complete nonlinear voltage dependence. It is clear that this dependence has an influence on volt-current measurement data.
4. Carbon wireframe nanosystems characterization As it is shown at Fig. 2, the CWNS geometry can be easy visualized with SEM, when these systems are grown at the apex of the metallic tip. The 150 mm chemical etching tungsten wire tips with 50 nm curving radius are used. 3D CWNS image might be obtained by the means of rotation of a tip vis-a -vis its symmetrical axis. Different experimental methods are used for detection of the electron properties. The single whisker separation from substrate assembly method is proposed at Ref. [6]. Whisker movement was arranged by electrostatic fields. Contact gold bonding areas for the whisker I(V) measurement were deposited by the focused electron beam method. This approach is technically complicated task and a contact between the whisker and the growth substrate is destroyed. As it is presented at Ref. [7], a great amount of whiskers was combined by the metallic electrode deposition and I(V) characteristics were averaged by the whisker assembly. It is clear that this method gives whisker assembly average characteristics and neglects fine characteristic structures. Another single semiconductor whisker I(V) measurement approach is used at Ref. [8]. Whiskers were grown by the molecular beam epitaxy and were placed perpendicular to the plane substrate. A contact to the whisker top was arranged by an atomic force microscope cantilever. Since the force cantilever impact could destroy a whisker, whiskers were fixed in a special grown matrix, so
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A.O. Golubok et al. / Physica E 44 (2012) 976–980
Fig. 2. 2D wireframe nanosystems grown at the tungsten tip SEM imaginations.
Fig. 3. Quantum graph (quantum network).
Fig. 4. Transition probability calculation in the simple quantum graph model.
there were only whisker tops above the matrix. Some percentage of gold whisker tops were split by tapping mode scanning. The cantilever was pressed to the whisker split top and I(V) characteristics were arranged. While the experiments were carried out at the atmosphere pressure, there was dielectric oxide layer at the whisker surface. In order to neglect this fact the sharp cantilever perforated this layer. In this way there are some problems with interpretation of the I(V) characteristics, it is due to the oxide layer perforation procedure, great local pressure at the whisker top, the metalsemiconductor contact Schottky barrier impact, lateral temperature drift, which is very important for tip-whisker positioning. Single nanowhiskers and carbon wireframe nanosystems growth technology at the tungsten tip apex is compatible with the tunnel spectroscopy method. It is usual that I(V) measurements in scanning tunnel microscope are arranged between a sharp metallic tip with defined properties and plane sample with undefined properties. This method allows to study sample density electron states features, for example it is possible to measure lateral band gap distribution at the semiconductor surface [9]. Since the STM measured tunnel current is convolution of tunnel junction bank properties, it is possible during I(V) measurement to investigate electron state density features of the sharp tunnel electrode if the plane electrode properties are known. An ultra high vacuum tunnel microscope (UHV TM) Omicron VT 650 was used for I(V) measurements . The tungsten tip with grown CWNS at its apex was arranged as the sharp tunnel electrode and high oriented pyrolytic graphite (HOPG) covered gold layer was arranged as a plane electrode. The experiments were carried out in the vacuum chamber with 10 10 mbar residual pressure, gold layer and tungsten tip prior temperature heating was about 160 1C. Fig. 5
A.O. Golubok et al. / Physica E 44 (2012) 976–980
Fig. 5. (a) Tungsten tip SEM image. (b) Reference I(V) characteristics of tunnel contact between the clean tungsten tip and the plane gold surface.
Fig. 6. (a) Tungsten tip with GaAs whisker SEM image. (b) Reference I(V) characteristic of tunnel junction between the plane gold surface and the GaAs whisker.
Fig. 7. (a) Tungsten tip with C whisker SEM image. (b) Reference I(V) characteristics of tunnel junction between the plane gold surface and the carbon whisker.
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A.O. Golubok et al. / Physica E 44 (2012) 976–980
shows I(V) characteristic of tunnel junction between the clean tungsten tip and the plane gold surface. As it was expected, there is a linear dependence at the (0,5; 0,5) V range. We have also measured I(V) characteristic of tunnel junction between the plane gold surface and the semiconductor GaAs whisker grown at the apex of the tungsten tip by the molecular beam epitaxy [10] (see Fig. 6). It is clear that they have the nonlinear character. This fact shows electron state band gap presence. The semiconductor band gap is defined by the tunnel current increase region and its value is about 1,5 eV. It is perfectly in agreement with known experimental and theoretical data. Therefore, the reference measurement results show that the whiskers and CWNG characterization proposed method is correct. Fig. 7 presents carbon whisker I(V) characteristic, whisker was grown at the apex of the tungsten tip. There is a nonlinear I(V) dependence, it is clearly showed that carbon whiskers have semiconductor conductivity properties.
5. Results and conclusions To sum up, the technology and the growth model of carbon whiskers and carbon wireframe nanosystems (CWNS) are proposed. CWNS creation at the apex of the W tip ensures convenient and adequate CWNS characterization with the use of SEM and UHV tunneling spectroscopy. The calculation method which is based on the quantum graph model is proposed for CWNS characterization.
The further low temperature experiments are planned for the detection of fine structure caused by the quant graphs on I(V) characteristics of tunneling junction.
Acknowledgments The authors are grateful to G.E. Tsyrlin and U.B. Sampsonenko, the Ioffe Physical Technical University of the RAS, for the preparation of GaAs whisker samples. The research was conducted with financial support of the Russian Ministry of Science and Education. References [1] Nikolai N. Ledentsov, Dieter Bimberg, Zh.I. Alferov, J. Lightwave Technol. 26 (11) (2008) 1540. [2] C.H. Jin, J.Y. Wang, Q. Chen, L.-M. Peng, J. Phys. Chem. B 110 (2006) 5423. [3] S.A. Chivilikhin, A.O. Golubok, I.S. Mukhin, NTV Ifmo 02 (66) (2010) 78. [4] P. Kuchment, Waves Random Media 12 (2002) R1. [5] M.A. Kokoreva, V.A. Margulis, M.A. Pyataev, Condens. Mater. 0912 (2009). [6] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [7] K. Haraguchi, T. Katsuyama, K. Hiruma, K. Ogawa, Appl. Phys. Lett. 60 (1991) 745. [8] P.A. Dementyev, M.S. Dunaevskii, Yu.B. Samsonenko, G.E. Cirlin, A.N. Titkov, Semiconductors 44 (5) (2010) 636. [9] A.O. Golubok, D.N. Davydov, S.A. Rykov, Ultramicroscopy 42–44 (1992) 878. [10] V.G. Dubrovskii, G.E. Cirlin, I.P. Soshnikov, A.A. Tonkikh, N.V. Sibirev, Yu.B Samsonenko, V.M. Ustinov, Phys. Rev. B 71 (2005) 205.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Creation and study of 2D and 3D carbon nanographs A.O. Golubok a,b, I.S. Mukhin c,n, I.U. Popov a, I.S. Lobanov a a
Saint-Petersburg State University of Information Technologies, Mechanics and Optics, Kronverksky Pr., 49, St. Petersburg 197101, Russian Federation Institute for Analytical Instrumentation of the Russian Academy of Sciences, Rizhsky Pr., 26, St. Petersburg 190103, Russian Federation Institution of the Russian Academy of Sciences, Saint-Petersburg Academic University—Nanotechnology Research and Education Centre RAS, Physics and Technologies Nanostructures, Khlopina St., 8/3, St. Petersburg 194021, Russian Federation b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2010 Accepted 22 October 2010 Available online 21 November 2010
The technology of nanowhiskers growth in the scanning electron microscope vacuum chamber have been applied for the creation of 2D and 3D carbon wireframe nanosystems (CWNS). The probability of electron transition has been calculated within simple quantum graph (quantum network) model. For the purpose of research of CWNS electron state density investigation it is proposed to grow CWNS on the tungsten tip apex and to measure current–voltage characteristics I(V) of Au plate – vacuum gap – CWNS W tip tunneling junction in the ultra high vacuum scanning tunneling microscope. Experiments show that proposed methodic approach gives adequate I(V) dependencies for metallic and semiconductor nanowhiskers at the room temperature. For detection of the peculiarities at I(V) tunneling junction characteristics caused by quantum graphs it is required to carry out measurements at low temperature conditions. & 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Technology of creation of carbon wireframe nanosystems
Creation, study and application of different solid state nanosystems based on 2D, 1D and 0D structural elements with quantum scale properties constitute an important direction of modern nanoscale science and technology. Semiconductor lasers based on quantum dots or quantum wire ensembles are an impressive example [1]. From the viewpoint of both the fundamental theory and its application it is interesting to create and study 2D and 3D quantum graph nanosystems containing 1D nanostructures as their edges. These systems are called wireframe nanosystems. According to the formal point of view it is a solution of the ¨ Schrodinger equation within 1D systems which are curved by the so-called hybrid manifold modulation with various dimensions. It can be electron state density features at the quantum graph nodes. Generally speaking, this quantum graph structure overpatching for example angle leaves changing can be used for the electron state density management. Therefore, the quantum graphs’ growth technology might provide for the opportunities of the well-defined properties’ nanosystem construction, including nanoelectronically and optoelectronically nonlinear elements. Apart from that, wireframe nanosystems can be used as elements of the micro- and nanomechanical devices, such as acceleration gages, cantilevers , liquid metal cathodes, field emission cathodes and nanotweesers.
Our technology of creation of carbon wireframe nanosystems is based on a single nanowhisker growth technology. The known nanowhisker growth methods are based on various physical– chemical phenomena, such as the molecular beam epitaxy, metalorganic chemical vapor deposition, electrolysis, laser ablation, focused beam electron deposition, etc. The focused electron beam material deposition method is used. As opposed to Ref. [2], the nanowhisker geometry growth monitoring is used. The experimental setup is presented in Fig. 1(a). A whisker growth substrate is placed in the scanning electron microscope vacuum chamber. The substrate might be planar or have a tip form. A carbon-containing target was placed near the substrate for acceleration of the growth processes. The 10 keV and 50 pA electron beam is focused at a whisker growth place. The electron beam illuminates both the substrate and the target surface. A 20–50 nm nanobump appears on the substrate surface after the focused beam short exposure. The nanowhisker growth is defined by an electron beam spatial movement. The nanowhisker growth direction is perpendicular to the electron beam. A focused beam shift trajectory defines a nanowhisker growth form. In that design the focused electron beam is a spatial drawing nanopan. It is important to mention that a minimal growth whisker dimension is bigger than an electron beam spot dimension (about 2 nm). That is why the electron beam scans a small 15 15 nm2 area during the exposure. The nanowhisker growth form is controlled by a bigger area scanning with secondary electrons during growth processes. This approach provides for carbon whisker growth opportunities
n
Corresponding author. Tel.: + 79516610258. E-mail address: [email protected] (I.S. Mukhin).
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.10.013
A.O. Golubok et al. / Physica E 44 (2012) 976–980
977
Fig. 1. (a) Whiskers and wireframe graph systems growth experimental setup: 1—the focused electron beam; 2—the substrate; 3—the carbon containinig target; 4—carbon ions; 5—grown whisker. (b) Simulation results of the whisker grown at the top of the tungsten tip.
and a whisker length is ten times larger than a typical lateral whisker size. It is important to mention that the whisker cross section does not have symmetric form with an extension along the beam direction. A simple whisker growth model is suggested in Ref. [3]. According to this model it is supposed that a positive carbon ion region is induced near the substrate surface by the focused electron 10–30 keV beam. The carbon ions interact with electron beam field. In this model there is a dedicated direction which is parallel to the electron beam axis. Carbon ions leave the substrate, oscillate near the beam axis and fill relatively small area near the electron beam (a surface growth size along the electron beam direction is bigger than the carbon ions localization area near the electron beam). This fact reflects asymmetric form of the whisker cross section. It is supposed that carbon ions neutralize at the substrate surface and absorb there under the secondary electrons impact. Model simulation shows that change of controlling parameters, for example, acceleration of voltage, electron beam current, beam diameter, distance between the target and the substrate, might impact whisker growth processes, change the whisker cross section and make more symmetrical form. Model simulation results are presented in Fig. 1(b). It should be observed that the whisker growth took place also in the vacuum chamber without the carbon target, but the growth rate in this condition is very insignificant. According to X-ray spectral microanalysis data, it was shown that a whisker material is carbon too. It is supposed that this fact is connected with carbon presence in vacuum chamber SEM residual atmosphere. Therefore, it is possible to create spatial carbon wireframe nanosystems (CWNS) by the focused electron beam shifting in the focal beam plane vis-a -vis the substrate and the substrate shifting with the SEM goniometer visa -vis the electron beam. Fig. 2 shows 2D wireframe nanosystems containing whiskers, those structures have been grown at the tungsten tip surface.
¨ 3. Schrodinger equation of 1D curvilinear systems Theoretical prediction of tunneling spectroscopy data through carbon whisker can be obtained using quantum graphs model [4]. The simple graph containing a loop formed by whiskers Ea and Eb attached to the tip Et and separated from the sample Es by the vacuum region Ev is considered. It is assumed that the electron motion in whiskers as well as in tip, vacuum and sample is quasi one-dimensional, therefore the configuration spaces for regions Ea, Eb and Ev is chosen to be finite segments of length la, lb and lv, respectively, and the configuration spaces for Es and Ev are halflines. The resulting quantum graph is shown in Fig. 3.
Electron scattering in similar systems was considered earlier (see Ref. [5] and references therein), however tunneling region and electric fields have been neglected up to now. The Hamiltonian of the considered system coincides with the ¨ convenient one-dimensional Schrodinger operator on wave functions vanishing outside one of segments Ee, eAI¼{a, b, v} hence the Hamiltonian can be obtained as the self-adjoint extension of the ¨ direct sum of Schrodinger operators on segments Ee defined on functions vanishing with derivatives at ends of segments onto functions on whole system satisfying to the Kirchhoff conditions (a continuous function is said to satisfy the Kirchhoff conditions, if the sum of external derivatives is equal to zero at every point). It is assumed that the scalar potential on whiskers is constant, and the potential on the vacuum region is approximated by a linear potential. The typical dependence of the transition probability from the tip to the sample on the applied energy is shown at Fig. 4 for the whiskers of 100 nm length, tunneling region of 6 nm width, potential barrier of 4 eV height under applied 0–8 V voltage, electron kinetic energy is equal to Fermi level energy. Fig. 4 shows that transition probability has complete nonlinear voltage dependence. It is clear that this dependence has an influence on volt-current measurement data.
4. Carbon wireframe nanosystems characterization As it is shown at Fig. 2, the CWNS geometry can be easy visualized with SEM, when these systems are grown at the apex of the metallic tip. The 150 mm chemical etching tungsten wire tips with 50 nm curving radius are used. 3D CWNS image might be obtained by the means of rotation of a tip vis-a -vis its symmetrical axis. Different experimental methods are used for detection of the electron properties. The single whisker separation from substrate assembly method is proposed at Ref. [6]. Whisker movement was arranged by electrostatic fields. Contact gold bonding areas for the whisker I(V) measurement were deposited by the focused electron beam method. This approach is technically complicated task and a contact between the whisker and the growth substrate is destroyed. As it is presented at Ref. [7], a great amount of whiskers was combined by the metallic electrode deposition and I(V) characteristics were averaged by the whisker assembly. It is clear that this method gives whisker assembly average characteristics and neglects fine characteristic structures. Another single semiconductor whisker I(V) measurement approach is used at Ref. [8]. Whiskers were grown by the molecular beam epitaxy and were placed perpendicular to the plane substrate. A contact to the whisker top was arranged by an atomic force microscope cantilever. Since the force cantilever impact could destroy a whisker, whiskers were fixed in a special grown matrix, so
978
A.O. Golubok et al. / Physica E 44 (2012) 976–980
Fig. 2. 2D wireframe nanosystems grown at the tungsten tip SEM imaginations.
Fig. 3. Quantum graph (quantum network).
Fig. 4. Transition probability calculation in the simple quantum graph model.
there were only whisker tops above the matrix. Some percentage of gold whisker tops were split by tapping mode scanning. The cantilever was pressed to the whisker split top and I(V) characteristics were arranged. While the experiments were carried out at the atmosphere pressure, there was dielectric oxide layer at the whisker surface. In order to neglect this fact the sharp cantilever perforated this layer. In this way there are some problems with interpretation of the I(V) characteristics, it is due to the oxide layer perforation procedure, great local pressure at the whisker top, the metalsemiconductor contact Schottky barrier impact, lateral temperature drift, which is very important for tip-whisker positioning. Single nanowhiskers and carbon wireframe nanosystems growth technology at the tungsten tip apex is compatible with the tunnel spectroscopy method. It is usual that I(V) measurements in scanning tunnel microscope are arranged between a sharp metallic tip with defined properties and plane sample with undefined properties. This method allows to study sample density electron states features, for example it is possible to measure lateral band gap distribution at the semiconductor surface [9]. Since the STM measured tunnel current is convolution of tunnel junction bank properties, it is possible during I(V) measurement to investigate electron state density features of the sharp tunnel electrode if the plane electrode properties are known. An ultra high vacuum tunnel microscope (UHV TM) Omicron VT 650 was used for I(V) measurements . The tungsten tip with grown CWNS at its apex was arranged as the sharp tunnel electrode and high oriented pyrolytic graphite (HOPG) covered gold layer was arranged as a plane electrode. The experiments were carried out in the vacuum chamber with 10 10 mbar residual pressure, gold layer and tungsten tip prior temperature heating was about 160 1C. Fig. 5
A.O. Golubok et al. / Physica E 44 (2012) 976–980
Fig. 5. (a) Tungsten tip SEM image. (b) Reference I(V) characteristics of tunnel contact between the clean tungsten tip and the plane gold surface.
Fig. 6. (a) Tungsten tip with GaAs whisker SEM image. (b) Reference I(V) characteristic of tunnel junction between the plane gold surface and the GaAs whisker.
Fig. 7. (a) Tungsten tip with C whisker SEM image. (b) Reference I(V) characteristics of tunnel junction between the plane gold surface and the carbon whisker.
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A.O. Golubok et al. / Physica E 44 (2012) 976–980
shows I(V) characteristic of tunnel junction between the clean tungsten tip and the plane gold surface. As it was expected, there is a linear dependence at the (0,5; 0,5) V range. We have also measured I(V) characteristic of tunnel junction between the plane gold surface and the semiconductor GaAs whisker grown at the apex of the tungsten tip by the molecular beam epitaxy [10] (see Fig. 6). It is clear that they have the nonlinear character. This fact shows electron state band gap presence. The semiconductor band gap is defined by the tunnel current increase region and its value is about 1,5 eV. It is perfectly in agreement with known experimental and theoretical data. Therefore, the reference measurement results show that the whiskers and CWNG characterization proposed method is correct. Fig. 7 presents carbon whisker I(V) characteristic, whisker was grown at the apex of the tungsten tip. There is a nonlinear I(V) dependence, it is clearly showed that carbon whiskers have semiconductor conductivity properties.
5. Results and conclusions To sum up, the technology and the growth model of carbon whiskers and carbon wireframe nanosystems (CWNS) are proposed. CWNS creation at the apex of the W tip ensures convenient and adequate CWNS characterization with the use of SEM and UHV tunneling spectroscopy. The calculation method which is based on the quantum graph model is proposed for CWNS characterization.
The further low temperature experiments are planned for the detection of fine structure caused by the quant graphs on I(V) characteristics of tunneling junction.
Acknowledgments The authors are grateful to G.E. Tsyrlin and U.B. Sampsonenko, the Ioffe Physical Technical University of the RAS, for the preparation of GaAs whisker samples. The research was conducted with financial support of the Russian Ministry of Science and Education. References [1] Nikolai N. Ledentsov, Dieter Bimberg, Zh.I. Alferov, J. Lightwave Technol. 26 (11) (2008) 1540. [2] C.H. Jin, J.Y. Wang, Q. Chen, L.-M. Peng, J. Phys. Chem. B 110 (2006) 5423. [3] S.A. Chivilikhin, A.O. Golubok, I.S. Mukhin, NTV Ifmo 02 (66) (2010) 78. [4] P. Kuchment, Waves Random Media 12 (2002) R1. [5] M.A. Kokoreva, V.A. Margulis, M.A. Pyataev, Condens. Mater. 0912 (2009). [6] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [7] K. Haraguchi, T. Katsuyama, K. Hiruma, K. Ogawa, Appl. Phys. Lett. 60 (1991) 745. [8] P.A. Dementyev, M.S. Dunaevskii, Yu.B. Samsonenko, G.E. Cirlin, A.N. Titkov, Semiconductors 44 (5) (2010) 636. [9] A.O. Golubok, D.N. Davydov, S.A. Rykov, Ultramicroscopy 42–44 (1992) 878. [10] V.G. Dubrovskii, G.E. Cirlin, I.P. Soshnikov, A.A. Tonkikh, N.V. Sibirev, Yu.B Samsonenko, V.M. Ustinov, Phys. Rev. B 71 (2005) 205.