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Modified laser ablation process for nanostructured thermoelectric nanomaterial fabrication
Applied Surface Science 254 (2007) 1211–1214 www.elsevier.com/locate/apsusc
Modified laser ablation process for nanostructured thermoelectric nanomaterial fabrication Yalin Lu *, R.J. Knize Laser and Optics Research Center (LORC), 2354 Fairchild Dr. 2A31, USAF Academy, CO 80840, United States Received 2 May 2007; received in revised form 20 June 2007; accepted 20 June 2007 Available online 28 June 2007
Abstract A modified pulsed laser deposition process was used to enhance the nanostructure generation inside Bi2Te3 nanocrystals. In this process, an additional femotosecond laser beam was used to add an energy shock on the ablated flume, which can result in rich nanostructures embedded inside Bi2Te3 nanocrystals. A large Si wafer was used to ‘freeze’ such nanostructures and to effectively collect such nanostructured nanocrystals for further processing. The generated nanocrystals were studied by X-ray diffraction and scanning transmission electron microscopy, and the results prove the existence of such embedded nanostructures. Such nanocrystals were also characterized electrically and thermally for the conductivity measurements. Published by Elsevier B.V. Keywords: Thermoelectric effect; Bi2Te3 nanocrystals; Nanostructured nanocrystals; Modified pulsed laser deposition process
1. Introduction A thermoelectric (TE) material’s performance can be characterized using a dimensionless quantity ZT, in which T is the temperature (in K) and Z the TE figure-of-merit (FOM) Z¼
S2 s : k
(1)
Here S is the TE power or Seebeck coefficient, s is the electrical conductivity and k is the thermal conductivity. Since that an increase in S normally implies a decrease in s because of the carrier density consideration, and since that an increase in s implies an increase in electronic contribution to k as given by the Wiedemann–Franz law, it will be very difficult to increase Z if only following a natural material approach. The well-known bulk TE material is the Bi2(1x)Sb2xTe3(1y)Se3y with a room temperature ZT around 1 for the composition of Bi0.5Sb1.5Te3 [1]. In the quest for new TE materials with large ZT, many approaches have been previously investigated. For example, an ideal TE material was suggested as ‘phonon-glass electroncrystal’, which means that the material should have a low * Corresponding author. Tel.: +1 719 333 7102; fax: +1 719 333 3182. E-mail addresses: [email protected], [email protected] (Y. Lu). 0169-4332/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.apsusc.2007.06.040
thermal conductivity as in a glass, and a high electrical conductivity as in a metallic crystal. In this connection, several bulk TE materials have been studied, including skutterudites and clathrates. In filled skutterudites, so far the highest ZT that has been found is 1.4 [2]. Most impressively, a ZT around 2.2 at 800 8C was recently found in bulk AgPb18SbTe20 [3]. It was argued that such high ZT may be related to the appearing of internal nanostructures, which actually coincidences well with Harman’s quantum dot embedded superlattices, in which a ZT around 2.0 was obtained [4]. Reducing dimensionality of the material structure is becoming one of the most efficient ways to increase ZT [5]. Low dimensionality can provide: (1) an approach to enhance the density of states (DOS) near the Fermi energy, leading to an enhancement of the Seebeck coefficient; (2) an opportunity to use these anisotropic Fermi surfaces in multi-valley cubic semiconductors; (3) an opportunity to increase boundary scattering of phonons at the barrier–well interfaces, without a large increase in electron scattering at the interface; and (4) an opportunity for increased carrier mobility at a given carrier concentration when quantum confinement are satisfied, so that modulation doping and d-doping can be utilized. Unfortunately, the way to use common nanocrystals (NCs) as starting materials for direct bulk TE material fabrication is not as effective as one expects. This is because of the common grain-coarsening effect
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easily occurring in regular sintering process, which can overgrow those NCs in size, and then reduce the anticipated phonon scattering effect. In this research, a new nanostructured nanocrystals (NSNCs) concept was suggested with the goal to enhance phonon scattering. Inside a NSNC, rich nanostructures are represented by structural discontinuities (interfaces), and are artificially created. A modified pulsed laser deposition (PLD) process was used for fabricating such NSNCs. Bi2Te3 was selected because of its popularity. The fabricated Bi2Te3 NSNCs before and after vacuum sintering were structurally examined, physically characterized, and compared to regular Bi2Te3 NCs fabricated under similar conditions but no laser irradiation. The results obtained indicate the effectiveness of such NSNC concept and the modified PLD for such TE materials, and the potential toward effectively improving their TE FOM. 2. Experimental Fabrication of Bi2Te3 NCs follows a modified laser ablation procedure using a 10 Hz femtosecond (fs) KrF 248 nm excimer laser having an output energy/pulse around 1 J. A commercial Bi2Te3 ceramics was used as a target. Modification of the operation includes: (1) using a large Si wafer (f > 3 in.) to collect the generated nanoparticles at a distance relatively far from the target (>10 cm). The Si wafer sits on an electrically powered heater, but was kept under relatively low temperature (50 8C) when performing the laser ablation. Low substrate temperature is used in order to prevent the nanostructure from relaxation happening under high temperature. The silicon wafers selected are undoped and have a known thermal conductivity of 149 W/m-K and an electrical conductivity of 0.012 (1 cm1; (2) using an additional ultraviolet (UV) laser beam to irradiate the laser ablated flume area (adding an energy shock to those nanoparticles inside the flume). This second laser beam was actually split out (1:4 in energy) from the main laser beam, and the pulses were synchronized to the main beam’s pulses by a time delay line external the system. Bi2Te3 NCs with and without the UV beam irradiation were fabricated under similar conditions, which will be used for further comparison. Bi2Te3 NCs to be used for electrical and thermal conductivity characterizations were remained in the deposition chamber, and underwent a subsequent high temperature sintering in high vacuum. After the vacuum sintering performed at a preset temperature 450 8C for hour, the collected Bi2Te3 NCs become film-like on the Si wafer. Using such film-like samples, electrical and thermal conductivities under a few different temperatures were measured. The electrical conductivity measurement was performed using a standard fourprobe method combined with a TE heater. A one-sided guarded-hot-plate apparatus was used to measure thermal conductivity. Details of this measurement system can be found in the literature [6]. Electrical and thermal conductivities of Bi2Te3 NCs with and without the UV beam irradiation were also compared. The fabricated Bi2Te3 NCs were structurally examined by X-ray diffraction analysis (XRD) and scanning transmission electronic microscopy (STEM).
Fig. 1. SEM image of Bi2Te3 NSNCs. The inset shows an image used for size analysis.
3. Results and discussion Fig. 1 shows a SEM image of the fabricated Bi2Te3 NSNCs (with the UV laser irradiation). The inset in Fig. 1 shows a scanning tunneling microscope (STM) image to be used for size analysis, which indicates an average nanoparticle size of 40 nm. Morphology of these vacuum sintered Bi2Te3 NC films on Si wafers is shown in Fig. 3. It is clear that those NSNCs are still nanoscale in size after sintering, which indicates that overgrowth of these nanocrystals was suppressed through this sintering condition. A slight melting on surface is also observable, which indicates an alight overheating. Bi2Te3 bulk crystal has a melting point around 580 8C. For Bi2Te3 NCs, their melting point should be lower. Crystalline structure of those NSNCs was revealed by XRD u/2u-scan characterization (Fig. 2 inset). The Bi2Te3 lattice’s (2 0 0) diffraction at 2u
Fig. 2. SEM image of the surface of sintered Bi2Te3 NSNCs on Si. The inset shows the XRD spectrum of the Bi2Te3 NSNCs’s (2 0 0) diffraction peak.
Y. Lu, R.J. Knize / Applied Surface Science 254 (2007) 1211–1214
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Fig. 3. HTEM images of Bi2Te3 NCs. (A and B) Ordered lattice structures favorably occurring in Bi2Te3 NCs without laser irradiation; (C and D) lattice discontinuities inside Bi2Te3 NSNCs.
29.58 can be clearly seen, which verifies the Bi2Te3 phase of those NCs. Evidence of lattice discontinuity introduced into Bi2Te3 NCs by laser irradiation was revealed using high resolution STEM. For comparison, lattices in Bi2Te3 NCs with or without laser irradiations were investigated. Fig. 3A and B show the lattice structures frequently occurring in Bi2Te3 NCs without laser irradiation, which are in a much better order than those from Bi2Te3 NSNCs as shown in Fig. 3C and D. Lattice discontinuities are more favorable to appear inside Bi2Te3 NSNCs. Rich internal interfaces occurring inside a NSNC are
Fig. 4. Electrical conductivity under different temperatures on sintered Bi2Te3 NSNCs and sintered Bi2Te3 NCs (no laser irradiation).
expected to contribute more to the phonon scattering enhancement. Using the Bi2Te3 NSNC films, both electrical and thermal conductivities were measured, and the results are shown in Figs. 4 and 5, respectively. Since that the electrical conductivity measurement was on the film layer’s surface and the film is very thick (>30 mm), we believe that only the film, not the Si substrate, contributed to the measured electrical conductivity. In thermal conductivity measurement, heat was crossing the
Fig. 5. Thermal conductivity under different temperatures on sintered Bi2Te3 NSNCs and sintered Bi2Te3 NCs (no laser irradiation). Result from Bi2Te3 single crystal is also listed.
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sample from Si substrate to the Bi2Te3 NSNCs film. Thermal contribution from Si substrate can be then deducted. Electrical conductivity of sintered Bi2Te3 NSNCs and NCs without laser irradiation shows a slight difference, and the NSNCs are slightly better. However, this difference is believed still in the error range of measurement (<5%). However, thermal results are very favorable because that an obvious thermal conductivity reduction in the sintered Bi2Te3 NSNCs was observed. This result was also compared to those from both sintered Bi2Te3 NCs and the bulk Bi2Te3 ceramics [7]. The result supports the viewpoint of enhancement of phonon scattering by the induced rich internal interfaces. 4. Conclusion A new NSNCs concept is suggested and a modified PLD was used to fabricate such Bi2Te3 NSNCs with a goal to enhance phonon scattering and thus to reduce the thermal conductivity. The fabricated NSNCs were studied by XRD and STEM, and the results verify the existence of such nanostructures. Electrical and thermal conductivity measure-
ments support the suggested NSNC concept’s validness and also the fabrication approach’s effectiveness. Acknowledgement The authors would like to acknowledge the support from the United States Air Force Office of Scientific Research (AFOSR) and Air Force Research Laboratories (AFRL). References [1] H.J. Goldsmid, Electronic refrigeration, in: European Conference on Thermophysical Properties, Pion Limited, London, 1986. [2] M. Fornari, D.J. Singh, Appl. Phys. Lett. 74 (1999) 3666. [3] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Science 303 (2004) 818. [4] T.C. Harman, P.J. Taylor, M.P. Walsh, B.E. LaForge, Science 297 (2002) 2229. [5] M.S. Dresselhaus, G. Dresselhaus, X. Sun, Z. Zhang, S.B. Cronin, T. Koga, Soviet physics, Phys. Solid State 41 (1999) 679. [6] B.J. Filla, Rev. Sci. Instrum. 68 (1997) 2822. [7] H.J. Goldsmid, Proc. Phys. Soc. B 69 (1956) 203.
Modified laser ablation process for nanostructured thermoelectric nanomaterial fabrication Yalin Lu *, R.J. Knize Laser and Optics Research Center (LORC), 2354 Fairchild Dr. 2A31, USAF Academy, CO 80840, United States Received 2 May 2007; received in revised form 20 June 2007; accepted 20 June 2007 Available online 28 June 2007
Abstract A modified pulsed laser deposition process was used to enhance the nanostructure generation inside Bi2Te3 nanocrystals. In this process, an additional femotosecond laser beam was used to add an energy shock on the ablated flume, which can result in rich nanostructures embedded inside Bi2Te3 nanocrystals. A large Si wafer was used to ‘freeze’ such nanostructures and to effectively collect such nanostructured nanocrystals for further processing. The generated nanocrystals were studied by X-ray diffraction and scanning transmission electron microscopy, and the results prove the existence of such embedded nanostructures. Such nanocrystals were also characterized electrically and thermally for the conductivity measurements. Published by Elsevier B.V. Keywords: Thermoelectric effect; Bi2Te3 nanocrystals; Nanostructured nanocrystals; Modified pulsed laser deposition process
1. Introduction A thermoelectric (TE) material’s performance can be characterized using a dimensionless quantity ZT, in which T is the temperature (in K) and Z the TE figure-of-merit (FOM) Z¼
S2 s : k
(1)
Here S is the TE power or Seebeck coefficient, s is the electrical conductivity and k is the thermal conductivity. Since that an increase in S normally implies a decrease in s because of the carrier density consideration, and since that an increase in s implies an increase in electronic contribution to k as given by the Wiedemann–Franz law, it will be very difficult to increase Z if only following a natural material approach. The well-known bulk TE material is the Bi2(1x)Sb2xTe3(1y)Se3y with a room temperature ZT around 1 for the composition of Bi0.5Sb1.5Te3 [1]. In the quest for new TE materials with large ZT, many approaches have been previously investigated. For example, an ideal TE material was suggested as ‘phonon-glass electroncrystal’, which means that the material should have a low * Corresponding author. Tel.: +1 719 333 7102; fax: +1 719 333 3182. E-mail addresses: [email protected], [email protected] (Y. Lu). 0169-4332/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.apsusc.2007.06.040
thermal conductivity as in a glass, and a high electrical conductivity as in a metallic crystal. In this connection, several bulk TE materials have been studied, including skutterudites and clathrates. In filled skutterudites, so far the highest ZT that has been found is 1.4 [2]. Most impressively, a ZT around 2.2 at 800 8C was recently found in bulk AgPb18SbTe20 [3]. It was argued that such high ZT may be related to the appearing of internal nanostructures, which actually coincidences well with Harman’s quantum dot embedded superlattices, in which a ZT around 2.0 was obtained [4]. Reducing dimensionality of the material structure is becoming one of the most efficient ways to increase ZT [5]. Low dimensionality can provide: (1) an approach to enhance the density of states (DOS) near the Fermi energy, leading to an enhancement of the Seebeck coefficient; (2) an opportunity to use these anisotropic Fermi surfaces in multi-valley cubic semiconductors; (3) an opportunity to increase boundary scattering of phonons at the barrier–well interfaces, without a large increase in electron scattering at the interface; and (4) an opportunity for increased carrier mobility at a given carrier concentration when quantum confinement are satisfied, so that modulation doping and d-doping can be utilized. Unfortunately, the way to use common nanocrystals (NCs) as starting materials for direct bulk TE material fabrication is not as effective as one expects. This is because of the common grain-coarsening effect
1212
Y. Lu, R.J. Knize / Applied Surface Science 254 (2007) 1211–1214
easily occurring in regular sintering process, which can overgrow those NCs in size, and then reduce the anticipated phonon scattering effect. In this research, a new nanostructured nanocrystals (NSNCs) concept was suggested with the goal to enhance phonon scattering. Inside a NSNC, rich nanostructures are represented by structural discontinuities (interfaces), and are artificially created. A modified pulsed laser deposition (PLD) process was used for fabricating such NSNCs. Bi2Te3 was selected because of its popularity. The fabricated Bi2Te3 NSNCs before and after vacuum sintering were structurally examined, physically characterized, and compared to regular Bi2Te3 NCs fabricated under similar conditions but no laser irradiation. The results obtained indicate the effectiveness of such NSNC concept and the modified PLD for such TE materials, and the potential toward effectively improving their TE FOM. 2. Experimental Fabrication of Bi2Te3 NCs follows a modified laser ablation procedure using a 10 Hz femtosecond (fs) KrF 248 nm excimer laser having an output energy/pulse around 1 J. A commercial Bi2Te3 ceramics was used as a target. Modification of the operation includes: (1) using a large Si wafer (f > 3 in.) to collect the generated nanoparticles at a distance relatively far from the target (>10 cm). The Si wafer sits on an electrically powered heater, but was kept under relatively low temperature (50 8C) when performing the laser ablation. Low substrate temperature is used in order to prevent the nanostructure from relaxation happening under high temperature. The silicon wafers selected are undoped and have a known thermal conductivity of 149 W/m-K and an electrical conductivity of 0.012 (1 cm1; (2) using an additional ultraviolet (UV) laser beam to irradiate the laser ablated flume area (adding an energy shock to those nanoparticles inside the flume). This second laser beam was actually split out (1:4 in energy) from the main laser beam, and the pulses were synchronized to the main beam’s pulses by a time delay line external the system. Bi2Te3 NCs with and without the UV beam irradiation were fabricated under similar conditions, which will be used for further comparison. Bi2Te3 NCs to be used for electrical and thermal conductivity characterizations were remained in the deposition chamber, and underwent a subsequent high temperature sintering in high vacuum. After the vacuum sintering performed at a preset temperature 450 8C for hour, the collected Bi2Te3 NCs become film-like on the Si wafer. Using such film-like samples, electrical and thermal conductivities under a few different temperatures were measured. The electrical conductivity measurement was performed using a standard fourprobe method combined with a TE heater. A one-sided guarded-hot-plate apparatus was used to measure thermal conductivity. Details of this measurement system can be found in the literature [6]. Electrical and thermal conductivities of Bi2Te3 NCs with and without the UV beam irradiation were also compared. The fabricated Bi2Te3 NCs were structurally examined by X-ray diffraction analysis (XRD) and scanning transmission electronic microscopy (STEM).
Fig. 1. SEM image of Bi2Te3 NSNCs. The inset shows an image used for size analysis.
3. Results and discussion Fig. 1 shows a SEM image of the fabricated Bi2Te3 NSNCs (with the UV laser irradiation). The inset in Fig. 1 shows a scanning tunneling microscope (STM) image to be used for size analysis, which indicates an average nanoparticle size of 40 nm. Morphology of these vacuum sintered Bi2Te3 NC films on Si wafers is shown in Fig. 3. It is clear that those NSNCs are still nanoscale in size after sintering, which indicates that overgrowth of these nanocrystals was suppressed through this sintering condition. A slight melting on surface is also observable, which indicates an alight overheating. Bi2Te3 bulk crystal has a melting point around 580 8C. For Bi2Te3 NCs, their melting point should be lower. Crystalline structure of those NSNCs was revealed by XRD u/2u-scan characterization (Fig. 2 inset). The Bi2Te3 lattice’s (2 0 0) diffraction at 2u
Fig. 2. SEM image of the surface of sintered Bi2Te3 NSNCs on Si. The inset shows the XRD spectrum of the Bi2Te3 NSNCs’s (2 0 0) diffraction peak.
Y. Lu, R.J. Knize / Applied Surface Science 254 (2007) 1211–1214
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Fig. 3. HTEM images of Bi2Te3 NCs. (A and B) Ordered lattice structures favorably occurring in Bi2Te3 NCs without laser irradiation; (C and D) lattice discontinuities inside Bi2Te3 NSNCs.
29.58 can be clearly seen, which verifies the Bi2Te3 phase of those NCs. Evidence of lattice discontinuity introduced into Bi2Te3 NCs by laser irradiation was revealed using high resolution STEM. For comparison, lattices in Bi2Te3 NCs with or without laser irradiations were investigated. Fig. 3A and B show the lattice structures frequently occurring in Bi2Te3 NCs without laser irradiation, which are in a much better order than those from Bi2Te3 NSNCs as shown in Fig. 3C and D. Lattice discontinuities are more favorable to appear inside Bi2Te3 NSNCs. Rich internal interfaces occurring inside a NSNC are
Fig. 4. Electrical conductivity under different temperatures on sintered Bi2Te3 NSNCs and sintered Bi2Te3 NCs (no laser irradiation).
expected to contribute more to the phonon scattering enhancement. Using the Bi2Te3 NSNC films, both electrical and thermal conductivities were measured, and the results are shown in Figs. 4 and 5, respectively. Since that the electrical conductivity measurement was on the film layer’s surface and the film is very thick (>30 mm), we believe that only the film, not the Si substrate, contributed to the measured electrical conductivity. In thermal conductivity measurement, heat was crossing the
Fig. 5. Thermal conductivity under different temperatures on sintered Bi2Te3 NSNCs and sintered Bi2Te3 NCs (no laser irradiation). Result from Bi2Te3 single crystal is also listed.
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Y. Lu, R.J. Knize / Applied Surface Science 254 (2007) 1211–1214
sample from Si substrate to the Bi2Te3 NSNCs film. Thermal contribution from Si substrate can be then deducted. Electrical conductivity of sintered Bi2Te3 NSNCs and NCs without laser irradiation shows a slight difference, and the NSNCs are slightly better. However, this difference is believed still in the error range of measurement (<5%). However, thermal results are very favorable because that an obvious thermal conductivity reduction in the sintered Bi2Te3 NSNCs was observed. This result was also compared to those from both sintered Bi2Te3 NCs and the bulk Bi2Te3 ceramics [7]. The result supports the viewpoint of enhancement of phonon scattering by the induced rich internal interfaces. 4. Conclusion A new NSNCs concept is suggested and a modified PLD was used to fabricate such Bi2Te3 NSNCs with a goal to enhance phonon scattering and thus to reduce the thermal conductivity. The fabricated NSNCs were studied by XRD and STEM, and the results verify the existence of such nanostructures. Electrical and thermal conductivity measure-
ments support the suggested NSNC concept’s validness and also the fabrication approach’s effectiveness. Acknowledgement The authors would like to acknowledge the support from the United States Air Force Office of Scientific Research (AFOSR) and Air Force Research Laboratories (AFRL). References [1] H.J. Goldsmid, Electronic refrigeration, in: European Conference on Thermophysical Properties, Pion Limited, London, 1986. [2] M. Fornari, D.J. Singh, Appl. Phys. Lett. 74 (1999) 3666. [3] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Science 303 (2004) 818. [4] T.C. Harman, P.J. Taylor, M.P. Walsh, B.E. LaForge, Science 297 (2002) 2229. [5] M.S. Dresselhaus, G. Dresselhaus, X. Sun, Z. Zhang, S.B. Cronin, T. Koga, Soviet physics, Phys. Solid State 41 (1999) 679. [6] B.J. Filla, Rev. Sci. Instrum. 68 (1997) 2822. [7] H.J. Goldsmid, Proc. Phys. Soc. B 69 (1956) 203.