Optoelectronic sensitization of carbon nanotubes by CdTe nanocrystals

We investigate the photoconductance of single-walled carbon nanotube-nanocrystal hybrids. The nanocrystals are bound to the nanotubes via molecular recognition. We find that the photoconductance of the hybrids can be adjusted by the absorption characteristics of the nanocrystals. In addition, the photoconductance of the hybrids surprisingly exhibits a slow time constant of about 1 ms after excitation of the nanocrystals. The data are consistent with a bolometrically induced current increase in the nanotubes caused by photon absorption in the nanocrystals.

Figure 1

(Color online) Four-step chemical scheme to build a nanotube-nanocrystal hybrid. (a) Step I: after an acid treatment, carbon nanotubes (CNTs) exhibit carboxyl groups at the sidewalls and the tips. (b) Step II: a semistable NHS-ester is bound to the CNTs. (c) Step III: biotinylized CNTs are obtained by binding biotin-PEO-amine to EDC. (d) Step IV: streptavidin-functionalized CdTe nanocrystals (NCs) bind to the biotin-functionalized CNTs. Graphs are not to scale (diameter of nanocrystals $\sim 10\text{nm}$, diameter of CNTs $\sim 1.2\text{nm}$, thickness of the streptavidin layer $\sim 5\text{nm}$).

Figure 2

(a) Transmission electron microscope (TEM) image of purified CNTs. (c) TEM image of nanotube-nanocrystal hybrids.

Figure 3

(a) Emission spectrum of dry nanotube-nanocrystal hybrids excited at $E_{\text{PHOTON}}=1.96\text{eV}$ at room temperature. Center maximum refers to fluorescence of the nanocrystals. CNTs produce a Raman signal as sharp Raman spikes (black triangles). (b) Corresponding Raman signal at room temperature. (c) Fluorescence and extinction spectrum of streptavidin-coated, unbound colloidal nanocrystals with a fluorescence maximum at $E_{\text{NC}}=1.59\text{eV}$ (white triangle) at room temperature. (d) Extinction spectrum of biotin and streptavidin solved in water at room temperature.

Figure 4

(Color online) (a) Atomic force microscope (AFM) image of an individual nanotube-nanocrystal hybrid contacted by source and drain electrodes (sample A). Inset: incident light is linearly polarized along direction $\widehat{E}$. Polarization angle $\phi$ can be controlled with respect to the orientation $\widehat{w}$ of the hybrid. (b) Typical photoconductance measurement of an ensemble sample made of nanotube-nanocrystal hybrids as a function of the laboratory time at $E_{\text{PHOTON}}=1.61\text{eV}$ (open circles) and $E_{\text{PHOTON}}=1.41\text{eV}$ (black circles). (c) Typical current-voltage characteristic of a single nanotube-nanocrystal hybrid without any laser excitation at a bath temperature $T_{\text{BATH}}$ of 5, 10, 15, 20, 30, 40, 50, 75, and 100 K.

Figure 5

[(a)–(d)] Normalized photoconductance of ensemble samples of steps I, II, III, and IV (as in Fig. 2) as a function of $E_{\text{PHOTON}}$ (at room temperature, $P=300\mu \text{W}$, $V_{\text{SD}}=300\text{mV}$, $f_{\text{CHOP}}=400\text{Hz}$, pulsed laser excitation).

Figure 6

[(a)–(d)] Normalized photoconductance of different ensemble samples as compared to Fig. 5 from the preparation steps I, II, III, and IV as a function of $f_{\text{CHOP}}$ at $E_{\text{PHOTON}}=1.38\text{eV}$, and 1.72 eV (at room temperature, $P=1.3\text{mW}$, $V_{\text{SD}}=70\text{mV}$, cw laser excitation).

Figure 7

Photoconductance data of individual nanotube-nanocrystal hybrids. (a) Normalized photoconductance of sample A as a function of $\phi$ ($f_{\text{CHOP}}=315\text{Hz}$, $E_{\text{PHOTON}}=1.38\text{eV}$, $P=300\text{nW}$, cw-laser excitation). (b) Normalized photoconductance of sample C as function of $E_{\text{PHOTON}}$ at perpendicular (circles) and parallel (open circles) polarization (at $T_{\text{BATH}}=4.5\text{K}$, $f_{\text{CHOP}}=317\text{Hz}$, $P=400\text{nW}$, $V_{\text{SD}}=100\text{mV}$, cw-laser excitation) and background noise without laser excitation (squares). [(c) and (d)] Chopper frequency dependence of the normalized photoconductance of sample C for parallel and perpendicular polarization at (c) $E_{\text{PHOTON}}=1.31\text{eV}$ and (d) $E_{\text{PHOTON}}=1.76\text{eV}$ and fit according to Eq. (1) (line) (at $T_{\text{BATH}}=7.2\text{K}$, $V_{\text{SD}}=100\text{mV}$, $P=400\text{nW}$, cw-laser excitation). (e) Chopper frequency dependence of the photoconductance of sample B (at $T_{\text{BATH}}=3.5\text{K}$, $E_{\text{PHOTON}}=1.75\text{eV}$, $V_{\text{SD}}=20\text{mV}$, and cw-laser excitation), fit (line) according to Eq. (1), and background noise without laser excitation (open circles). (f) Photoconductance of sample D at room temperature ($f_{\text{CHOP}}=480\text{Hz}$, $P=50\mu \text{W}$, and pulsed laser excitation).

Figure 8

(Color online) Model for different energy and heat transfer mechanisms within the nanotube-nanocrystal hybrid and the substrate.