Current oscillations, switching, and hysteresis in CdSe nanorod superlattices

CdSe quantum rod (QR) arrays show striking and anomalous transport properties below $300\mathrm{K}$. Strong and reproducible nonlinearities, including current oscillations with dc voltage, are observed in the current-voltage characteristics above a temperature-dependent threshold. The observations are tentatively interpreted as due to resonant tunneling of charge carriers between adjacent QR wells via localized states in insulating barriers.

Figure 1

TEM images of a CdSe QR sample, showing a narrow size distribution of $\sim 5\%$. Counting over 300 QRs resulted in an average length of $18\pm 1\mathrm{nm}$ and an average diameter of $5.2\pm 0.3\mathrm{nm}$.

Figure 2

(Color online) Typical $I\text{−}{V}_{\mathit{\text{bias}}}$ characteristics as a function of $T$ for device A $(l=1\mu \mathrm{m})$. Only forward bias traces are shown for clarity. The inset displays a typical hysteretic loop. The arrows show the direction of the $V_{\mathit{\text{bias}}}$ sweep.

Figure 3

(Color online) Typical $I\text{−}{V}_{\mathit{\text{bias}}}$ characteristics as a function of $T$ for device B $(l=2\mu \mathrm{m})$. Only forward bias traces are shown for clarity. Notice the multiple switching events at $V_{\mathit{\text{bias}}}\sim V_T$ for curves taken at 250 and $260\mathrm{K}$. The inset shows the temperature dependence of the zero-bias conductance $G$.

Figure 4

(Color online) Temperature dependence of the threshold voltage $V_T$ for devices A (diamonds) and B (circles).

Figure 5

(Color online) $I\text{−}{V}_{\mathit{\text{bias}}}$ characteristics at $250\mathrm{K}$ for device A. The upper inset is an expanded view of the $I\text{−}{V}_{\mathit{\text{bias}}}$ in a limited $V_{\mathit{\text{bias}}}$ range, showing the regular periodicity of the current oscillations. The lower inset shows the periodicity $\Delta V_{\mathit{\text{bias}}}$ of current oscillations as a function of $T$. $\Delta V_{\mathit{\text{bias}}}$ was determined from an average of over 50 periods.

Figure 6

TEM images of typical CdSe QR superlattices formed when CdSe QRs are deposited by slow solvent evaporation method on thin films of amorphous carbon supported by a copper grid. CdSe QRs have strong preference for self-assembly into separate closed-packed “stacks,” in which QRs align with their long axis parallel to each other. The long tracks of QR stacks may align parallel to each other, resulting in smectic ordered superstructures.

Figure 7

(Color online) Schematic conduction band profile of a semiconductor (CdSe QR)-insulator (TOPO) superlattice for several values of the average potential per period $\Delta V_{\mathit{\text{bias}}}$. (a) Zero bias. (b) Direct tunneling through ground state $E_1$; arrows indicate electron transport. (c) First resonant tunneling occurs through localized state in the insulating barrier. (d) Second resonant tunneling.