Strong-electric-field effects and antenna resonances in single-wall carbon nanotube films

Transcription

1 Strong-electric-field effects and antenna resonances in single-wall carbon nanotube films Dalius Seliuta Center for Physical Sciences and Technology, Vilnius, Lithuania

2 Liudas Subačius, Irmantas Kašalynas, Gintaras Valušis Center for Physical Sciences and Technology, Vilnius, Lithuania Vitaly Ksenevich Belarus State University, Nezalezhnastsi Ave. 4, Minsk, Belarus Mihail Shuba, Olesia Paddubskaja, Polina Kuzhir, Gregory Slepyan Sergey Maksimenko Institute for Nuclear Problems, Bobruiskaya 11, Minsk, Belarus

3 Outline Introduction (SWCNT infrared detectors). THz photoconductivity of free-standing SWCNT fibers. THz photoconductivity of SiO2-SWCNT fibers Electrical conductivity of SWCNT layers in strong electric fields

4 Single-wall carbon nanotube (SWCNT) n 1 n 2 = 3q + p: p = 0 - metallic p = 1, 2 - semiconducting

5 Taken from: Carbon 42, 919 (2004) CNT near IR detectors

6 Taken from: Science 312, 413 (2006) CNT near IR detectors 50 K

7 Taken from: Appl. Phys. Lett. 92, (2008) CNT far IR detectors

8 THz photoconductivity of free-standing SWCNT fibers.

9 V. Ksenevich et al., J. Appl. Phys. 104, (2008) Free-standing SWCNT fibers R, Ω T R R0 exp n = 1 + d M T 1 n ; R, W T, K 50 R, Ω T~80 K T -1/4, K -1/4 T T T, K 0 1 T1 exp ; 0 R R T T AV 2 e k (2m ) B 8 0 AV0 2 e k w B 2 e 3/ 2 0 1/ 2 w 2 ;

10 V. Ksenevich et al., J. Appl. Phys. 104, (2008) Photoconductivity in THz region Signal, mv 5 Signal, mv K 14.2 K K K 20 K 40 K 30 K 50 K K 30 K t, ms Photosignal, mv t, ms Laser pulse: 0.6 Duration 100 µs Repetinion rate 30 Hz Frequency 2.5 THz T, K

11 THz photoconductivity of SiO 2 -SWCNT fibers

12 D. Seliuta et al., Appl. Phys. Lett. 97, (2010) SiO 2 -CNT fibers Silver paste SiO 2 -CNT fiber Glass R, kw 200 R, kw exp TM R R0 T T, K 0,2 0,3 0,4 0,5 0,6 0,7 0,8 T -1/4, K -1/4

13 D. Seliuta et al., Appl. Phys. Lett. 97, (2010) THz photoconductivity of SiO 2 -CNT fibers Bolometric signal: du dt AP t, B U B Photosignal: U ph = B P(t) 8 6 Signal, mv 4 2 Experiment Hopping + Bolometric Bolometric signal THz induced hopping 4K 2.5 THz 0 0,5 1,0 1,5 Time, ms

14 D. Seliuta et al., Appl. Phys. Lett. 97, (2010) Properties of the photoconductive signal U ph, mv THz 0.76 THz U ph, mv Normalized photoconductive signal, a.u Frequency, THz 4 K K 2.5 THz T, K P, mw

15 M. Shuba et al., Diamond and Related Materials, p. 36 (2012) THz absorption in CNT layer Re( eff ) Re( eff ) 20 A , N L 0 = 0.5 m L 0 = 1 m L 0 = 2 m eff eff 1, LN L L 1 3 exp L L 2Δ j 0 j i0 eff 1 2 j dl Δ 0. 3L 0 j (m, n) 0, f (THz) A 1 R T

16 M. Shuba et al., Diamond and Related Materials, p. 36 (2012) Absorption and photoconductivity in CNT layer 0,12 0,10 0,08 A 0,06 0,04 0,02 L 0 =2 µm T=4K 4 Normalized photoconductive signal f(thz)

17 Electrical conductivity of SWCNT layers in strong electric fields

18 Pulsed measurement setup L GHz U I Sample 100 kω L 2 L 1 U R RS 0 1kV Switch ( Hg Relay) U R /2 U T U I 50 Ω

19 Time dependence of current and voltage pulses Voltage (V) Current (ma) Conductive stripes: width 5 μm, spacing 5 μm SWCNT layer thickness 100 nm Time (ns) Up to 100 kv/cm

20 D. Seliuta et al., J. Appl. Phys. 113, (2013) I-V characteristics of SWCNT layer 0.03 Current (A) K 50 K 100 K 150 K 200 K 250 K 300 K Current (A) Voltage (V) Voltage (V)

21 D. Seliuta et al., J. Appl. Phys. 113, (2013) R-E characteristic of SWCNT layer (variable range hopping model) e E C L k T 3,0 Resistance (kw) E c T=4K E c (kv/cm) 2,5 2,0 1, T (K) E (V/cm) E C k T e L

22 D. Seliuta et al., J. Appl. Phys. 113, (2013) R-T dependencies of SWCNT layer (fluctuation induced tunneling model) R (kw) kv/cm 3 kv/cm kv/cm 20 kv/cm 80 kv/cm T (K) 0 TC T T e S

23 D. Seliuta et al., J. Appl. Phys. 113, (2013) R-T dependencies of SWCNT layer (modified fluctuation induced tunneling model) R T E, kv/cm AT B e A, Ω/K TC T T S B, Ω T C, K T S, K R (kw) kv/cm 3 kv/cm 6 kv/cm kv/cm 80 kv/cm T (K)

24 D. Seliuta et al., J. Appl. Phys. 113, (2013) R-T dependencies of acid-treated SWCNT layer 200 R (kw) kv/cm 3 kv/cm 6 kv/cm 20 kv/cm 80 kv/cm T (K)

25 Conclusions At low temperatures, terahertz photoconductivity is related with activation of the localized carriers by the terahertz photons. Fast photosignal can be distinguished from the thermal component using pulsed excitation. Due to finite length of CNT antenna resonances are observed in photoconductivity. Resonant frequencies depend mainly on the CNT length. Heat sink from the CNT layer to the substrate allows for attenuation of the thermal component which is advantageous for detection of fast terahertz pulses. Crossover from semiconducting behavior to metallic behavior at electric field strength around 3 kv/cm is explained in terms of variation of transparency of the insulating barriers with the electric field strength.