Surface Modifications by Field Induced Diffusion

Abstract

By applying a voltage pulse to a scanning tunneling microscope tip the surface under the tip will be modified. We have in this paper taken a closer look at the model of electric field induced surface diffusion of adatoms including the van der Waals force as a contribution in formations of a mound on a surface. The dipole moment of an adatom is the sum of the surface induced dipole moment (which is constant) and the dipole moment due to electric field polarisation which depends on the strength and polarity of the electric field. The electric field is analytically modelled by a point charge over an infinite conducting flat surface. From this we calculate the force that cause adatoms to migrate. The calculated force is small for voltage used, typical 1 pN, but due to thermal vibration adatoms are hopping on the surface and even a small net force can be significant in the drift of adatoms. In this way we obtain a novel formula for a polarity dependent threshold voltage for mound formation on the surface for positive tip. Knowing the voltage of the pulse we then can calculate the radius of the formed mound. A threshold electric field for mound formation of about 2 V/nm is calculated. In addition, we found that van der Waals force is of importance for shorter distances and its contribution to the radial force on the adatoms has to be considered for distances smaller than 1.5 nm for commonly used voltages.

Neuromorphic behavior in percolating nanoparticle films

“…We then identify those gaps with electric fields larger than a chosen threshold (here E th = 0.9) [45] which are then replaced with a large conductor (G Ohmic = 10 −1 ) with probability P ↑ . This process simulates the formation in the tunnel gap of an atomic wire, which occurs due to either electric-field-induced surface diffusion (EFISD) or electric-field-induced evaporation (EFIE) [27,[46][47][48]. We then recalculate the conductance of the network G. At the next voltage step the process is repeated; the voltage ramps continue until the system conductance converges-typically to within 0.1% of the value on the previous step.…”

“…On a microscopic scale P ↑ accounts for different local environments around each tunnel gap and the time taken for electric field effects to result in formation of the atomic wire [46][47][48], a process which is not completely understood.…”

“…6(c)]. The average size of the gaps decreases with increasing p [33] and together with the smaller number of gaps this means the average electric field in each gap is higher, which means that electric field driven switching [27,[46][47][48] is more likely for higher p. In contrast, we expect that for coverages lower than those studied here the network will be simplified and become similar to a random system of dopants [33,49]; this regime is not experimentally accessible in the nanoparticle films of interest because the current flowing falls below the noise threshold.…”

“…These tunnel gaps are memristive in that the device history determines its state. Switching occurs at a well-defined threshold value (of the electric field [46][47][48]) but unlike the the memristive elements in Refs. [1,3] the junction cannot be switched back to the low conduction state by simply reversing the polarity of the bias voltage.…”

Quantized Conductance and Switching in Percolating Nanoparticle Films

“…[25] which shows that high electric fields are required for electric field induced evaporation (EFIE - Fig. 1 (e)) and electric field induced surface diffusion (EFISD - Fig.…”