Charge transport and interdot coupling tuned by the tunnel barrier length in assemblies of nanoparticles surrounded by organic ligands

Low-temperature electrical measurements were performed on arrays of metallic nanoparticles coated by alkylamines. The length of the alkyl molecules was varied (18, 12, and four carbons), while the size of the metallic nanoparticles was fixed at 10.3 nm. This allowed us to investigate the effect of interdot coupling on the electronic transport. We show that in the Coulomb-blockade regime, the temperature and bias dependence of the resistivity are governed by the length of the organic barriers and are well described by theories that consider inelastic cotunneling to be the main mechanism of interparticle transfer. Indeed, quantitative agreement between theory and experiments is obtained when considering the interparticle distance deduced from transmission electron microscopy with respect to the relevant characteristic energies (activation temperature and charging energy).

Figure 1

TEM micrographs of Co nanoparticles of (a) C4, (b) C12, and (c) C18 samples. (d) Size distribution of the Co NPs and barrier length distributions for C12 and C18.

Figure 2

(a) SEM micrograph of the NPs (C12) trapped between Au fingerlike electrodes. (b) Scheme and dimensions of the electrodes. (c), (d) TEM cross-sectional micrographs of the devices (C12). (e) Autocorrelation treatment of (d) as well as (f) its analysis using a Gaussian function centered at a mean nearest-neighbor distance of 12.5 ± 1.0 nm.

Figure 3

(a) Room-temperature resistance measured on seven different samples of C4, C12, and C18. (b) Measured resistance vs temperature characteristics (symbols) and their adjustments (lines) in the framework of the inelastic cotunneling model [Eq. (1)]. (c) $I$($V$) characteristics measured at $T$ = 2 K and (d) their fits using Eq. (2).

Figure 4

(a) Charging energies as a function of the barrier length: Theoretical values according to Eq. (4) (black dotted line), Eq. (5) (red dashed line), and Eq. (3) (green solid line); experimental data (open circles). (b) Calculated ratio of the charging energy of an isolated NP [Eq. (4)] over that of coupled NPs [Eq. (3)] vs radius and barrier length.