Measuring charge trap occupation and energy level in CdSe/ZnS quantum dots using a scanning tunneling microscope

We use a scanning tunneling microscope to probe single-electron charging phenomena in individual CdSe/ZnS (core/shell) quantum dots (QDs) at room temperature. The QDs are deposited on top of a bare Au thin film and form a double-barrier tunnel junction (DBTJ) between the tip, QD, and substrate. Analysis of room-temperature hysteresis in the current-voltage $\left(IV\right)$ tunneling spectra, is consistent with trapped charge(s) presenting an additional potential barrier to tunneling, a measure of the Coulomb blockade. The paper describes the first direct electrical measurement of the trap-state energy on individual QDs. Manipulation of the charge occupation of the QD, verified by measuring the charging energy, $\left(61.4\pm 2.4\right)\text{meV}$, and analysis of the DBTJ, show trap states $\sim 1.09\text{eV}$ below the QD conduction-band edge. In addition, the detrapping time, a measure of the tunneling barrier thickness, is determined to have an upper time limit of 250 ms. We hypothesize that the charge is trapped in a quantum-dot surface state.

Figure 1

(Color online) Depictions of (a) STM tip over a QD on a Au-film substrate and (b) equivalent circuit where $C_1$, $R_1$ and $C_2$, $R_2$ are the tunnel barriers between the tip-QD and QD-substrate, respectively. (c) Energy-band diagram of a CdSe/ZnS QD with ligand capping layer on a Au-film substrate with both the substrate and STM tip grounded.

Figure 2

(Color online) Scanning probe images of CdSe/ZnS QD on Au. Left column (a), (c), and (e) are as-deposited samples. Right column (b), (d), and (f) are images of samples annealed at $120°\text{C}$ for 1 h in low vacuum (8 Torr). Top row (a) and (b) shows STM topography images, with a marked increase in clarity for the annealed samples. Middle row (c) and (d): AFM topography images show as-deposited and annealed average QD height of 6.6 nm and 6.1 nm, respectively. Bottom row (e) and (f): AFM phase images show distinct contrast between the QD and Au sample areas.

Figure 3

Tunneling spectra are taken in sequential pairs, first a decreasing substrate bias $\left(V_B\right)$ sweep 1, immediately followed by an increasing bias sweep 2. Point $\mathbf{A}$ is the detrap potential, the $V_B$ held for either 0 or 250 ms, before making a measurement. Points ${\text{edge}}_1$ and ${\text{edge}}_2$ indicate the conduction-band edge of the QD and ${\text{edge}}_2{\text{-edge}}_1$ is the charging energy, $E_c$. $IV$ inset shows a single discharging event. N and N-1 label the number of trapped charges on the QD.

Figure 4

Distributions of charging-energy measurements for three discharging conditions labeled in the figure. Histograms are composed of $k$ number of $E_c$ measurements and have a Gaussian peak, $\Gamma$, with 95% confidence bounds at (a) $k=722$, $\Gamma =\left(19\pm 10\right)\text{meV}$; (b) $k=1017$, $\Gamma =\left(1\pm 4\right)\text{meV}$; and (c) $k=692$, $\Gamma =\left(63\pm 6\right)\text{meV}$.

Figure 5

$E_c$ as a function of substrate bias (detrap potential, bottom axis) and bias drop between the QD and the Au substrate ($E_{F,\text{QD}}-E_{F,\text{Au}}$, top axis) for a wait time of 0 s (gray circles) and 250 ms (black triangles). $\Delta E_c$ is the difference between the weighted averages of the 250 ms data, above and below DP $\sim 1.55\text{V}$ $\left(E_{F,\text{QD}}-E_{F,\text{Au}}\sim 262\text{meV}\right)$.

Figure 6

Distributions of conduction-band edge energy, labeled ${\text{edge}}_1$ and ${\text{edge}}_2$ (see Fig. 3), for (a) low and (b) high DP at 250 ms wait time. At high DPs, the distributions of ${\text{edge}}_1$ and ${\text{edge}}_2$ are separated by $\left(66\pm 13\right)\text{meV}$, in contrast to the low DP separation of $\left(12\pm 16\right)\text{meV}$. The labels N and N-1 are indicative of the number of excess charges on the QD.

Figure 7

(Color online) Charging-energy $\left(E_c\right)$ calculation. (a) Schematic of capacitance between the QD and substrate, tip, and neighboring QDs. (b) An example of a probed QD and five to six nearest neighbors in a hexagonal close pack structure (STM image). (c) $E_c$ contour plot for a trapped charge at the core/shell interface, plotted versus ligand length and number of neighboring quantum dots. Measured $E_c$, $\left(61.4\pm 2.4\right)\text{meV}$, is highlighted on the contour plot.

Figure 8

Tunneling diagram for detrapping a charge from the QD at $V_B>1.55\text{V}$. At $V_B=0\text{V}$, the Fermi level of Au and the W tip are in equilibrium and lie slightly above the surface-state band, trapping charge(s) on the QD. The distribution of $V_B$ over the two tunnel gaps ($\eta \ast V_B$ over the QD-Au gap) causes the Fermi level of the Au to drop slightly from its equilibrium energy, thus exposing empty states on the Au surface to the trapped charge.