Electric Field Control of Molecular Charge State in a Single-Component 2D Organic Nanoarray

Abstract

Quantum dots (QD) with electric-field-controlled charge state are promising for electronics applications, e.g., digital information storage, single-electron transistors, and quantum computing. Inorganic QDs consisting of semiconductor nanostructures or heterostructures often offer limited control on size and composition distribution as well as low potential for scalability and/or nanoscale miniaturization. Owing to their tunability and self-assembly capability, using organic molecules as building nanounits can allow for bottom-up synthesis of two-dimensional (2D) nanoarrays of QDs. However, 2D molecular self-assembly protocols are often applicable on metals surfaces, where electronic hybridization and Fermi level pinning can hinder electric-field control of the QD charge state. Here, we demonstrate the synthesis of a single-component self-assembled 2D array of molecules [9,10-dicyanoanthracene (DCA)] that exhibit electric-field-controlled spatially periodic charging on a noble metal surface, Ag(111). The charge state of DCA can be altered (between neutral and negative), depending on its adsorption site, by the local electric field induced by a scanning tunneling microscope tip. Limited metal–molecule interactions result in an effective tunneling barrier between DCA and Ag(111) that enables electric-field-induced electron population of the lowest unoccupied molecular orbital (LUMO) and, hence, charging of the molecule. Subtle site-dependent variation of the molecular adsorption height translates into a significant spatial modulation of the molecular polarizability, dielectric constant, and LUMO energy level alignment, giving rise to a spatially dependent effective molecule–surface tunneling barrier and likelihood of charging. This work offers potential for high-density 2D self-assembled nanoarrays of identical QDs whose charge states can be addressed individually with an electric field.

Synthesis and Local Probe Gating of a Monolayer Metal‐Organic Framework

“…Each molecule has slightly varying adsorption environment (e.g., due to the moiré pattern on graphene on Ir(111) which is known to give rise to a work function modulation of a couple of hundred meV [58][59][60][61] ), varying the exact on-set bias of the charging. [42,45,48,50] The elliptical rings are mostly around the DCA molecules, which is consistent with the DFT results that the band above the Fermi level (which is pulled below E F at negative bias) has most of its density on the DCA molecules. These local charging features demonstrate that electron-electron interactions (characterized by the Coulomb charging energy, here ≈0.6 eV) are significant and they can be expected to be of a similar magnitude compared to the overall band width of the Cu-DCA network (here several hundreds of meV).…”

“…Interestingly, the STS measured on the center of the DCA molecule (black curve in Figure 2b) shows two sharp dips around −0.6 and −1.2 V; the STS on top of the Cu atom (green curve in Figure 2b) shows a small dip around −0.6 V and a sharp peak around −1.2 V; the end of the long axis of the DCA molecule (blue curve in Figure 2b) shows a sharp peak around −0.6 V and a tiny dip around −1.2 V. The peaks/dips at these two bias values are attributed to the typical charging features, where the charge state of the molecule under the tip changes due to the tip-induced local electric field. [38][39][40][41][42][43][44][45][46][47][48][49][50] We will discuss the details of these charging features in Figure 4. The spectra of the MOF on the step edge of the underlying Ir(111) substrate (Figure S12, Supporting Information) is consistent with the one on the flat area (Figure 2), indicating that the electronic properties of the MOF are effectively decoupled from the metal substrate by the graphene layer.…”

“…Charging behavior has been studied in detail in the case of single molecules or self-assembled molecular monolayers on coinage metal surfaces such as Au(111) [42] and Ag(111) [47,48] and more frequently found when the molecule is decoupled from the substrate by an ultra-thin film such as Al 2 O 3 , [38][39][40][41] graphene [46,49] and hexagonal boron nitride. [43][44][45]50] It can be explained by considering the tip-molecule-substrate system as a doublebarrier tunnel junction (DBTJ) as illustrated in Figure 4a,b.…”

“…Bringing the tip closer to the molecule increases α and, consequently, the charging features shift toward the Fermi level monotonously with decreasing tip-molecule distance (shown in Figure 4c), which is consistent with previous studies. [38,48,49] The charging features shift by about 8.8 mV Å −1 upon reducing the tip-sample distance. On the other hand, the kagome band position at the positive bias barely changes with different tipmolecule distances, which can also be seen from Figure 4c.…”

Building Functional Memories and Logic Circuits with 2D Boron Nitride

“…The rapid progress in BN and other 2D material‐based heterostructures provides strong motivation for further applications including exploration of the electrical and optoelectronic properties. [ 25,82,208,251,262 ] The negative photoconductance is also an interesting phenomenon for electronic devices.…”