Controlling Isomerization of Photoswitches to Modulate 2D Logic‐in‐Memory Devices by Organic–Inorganic Interfacial Strategy

Abstract Logic‐in‐memory devices are a promising and powerful approach to realize data processing and storage driven by electrical bias. Here, an innovative strategy is reported to achieve the multistage photomodulation of 2D logic‐in‐memory devices, which is realized by controlling the photoisomerization of donor–acceptor Stenhouse adducts (DASAs) on the surface of graphene. Alkyl chains with various carbon spacer lengths (n = 1, 5, 11, and 17) are introduced onto DASAs to optimize the organic–inorganic interfaces: 1) Prolonging the carbon spacers weakens the intermolecular aggregation and promotes isomerization in the solid state. 2) Too long alkyl chains induce crystallization on the surface and hinder the photoisomerization. Density functional theory calculation indicates that the photoisomerization of DASAs on the graphene surface is thermodynamically promoted by increasing the carbon spacer lengths. The 2D logic‐in‐memory devices are fabricated by assembling DASAs onto the surface. Green light irradiation increases the drain–source current (I ds) of the devices, while heat triggers a reversed transfer. The multistage photomodulation is achieved by well‐controlling the irradiation time and intensity. The strategy based on the dynamic control of 2D electronics by light integrates molecular programmability into the next generation of nanoelectronics.


Materials and reagents
All the chemicals and reagents were directly used without further purification. The Dow Chemical Company.

General methods
Density functional theory (DFT) simulations were performed using CP2K (http://www.cp2k.org) [1] based on the mixed Gaussian and plane-wave scheme [2] and the Quickstep module. [3] The calculation used Perdew-Burke-Ernzerhof (PBE) exchange correlation functional [4] , and molecularly optimized short range Double-Zeta-Valence plus Polarization basis set [5] with Goedecker-Teter-Hutter pseudopotentials [6] (DZVP-MOLOPT-SR-GTH). The plane-wave energy cutoff was 400 Ry, and a Grimme's dispersion correction with Becke-Johnson damping (D3BJ) dispersion correction [7] was applied. The calculation was performed on Gamma point only without symmetry constraint. Structural optimization was performed using the Broyden-Fletcher-Goldfarb-Shannon (BFGS) optimizer until the maximum force fell below 0.00045 Ry/Bohr (0.011 eV/Å). The finite displacement method was used for the phonon calculation, with incremental displacement of 0.01 Bohr (0.0053 Å). HOMO and LUMO diagrams were drawn via VMD (1.9.3) software, where the isovalue of HOMO was 0.02 a.u., and that of LUMO was 0.015 a.u. 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance 400 MHz spectrometer. UV/vis transmittance and absorption spectra were measured on a Shimazu UV-2600. To find out how much excitation light was absorbed by the films of DASAs, the transmittance spectra were transformed into absorbance spectra by equation (1) where A, T, I0 and It represent the absorbance, transmittance, intensity of incident light and intensity of transmission light, respectively. The films of DASA-6C, DASA-12C and DASA-18C show similar absorbance in the visible light region, which is with good accordance to the results of thickness.
X-ray diffraction (XRD) data were collected in the angular range of 2θ =2 ̊90 ̊ with a Bruker D8 Advance X-ray diffractometer.
The elemental mapping was performed on a field-emission scanning electron microscope (SEM) (Carl Zeiss GeminiSEM 300) equipped with an energy-dispersive X-ray spectrometer (EDS). Samples were scattered on silicon wafers and metal sprayed.
The morphology on the films surface was determined with a transmission electron microscopy (TEM) (FEI Tecnai F20) microscope. Samples were prepared on a carboncoated copper grid.
The morphology on the films surface was determined with an atomic force microscope (AFM) (Bruker Multimode 8) microscope. Samples were spin-coated on the silicon wafer and graphene device.
Drain-source current-threshold voltage (Ids-Vg) curves for the 2D logic-in-memory devices were tested by Pro Plus FS-Pro.
The LED light source with the emitted wavelength at 520 nm was purchased from Zhongjiao Jinyan Systems. The output intensity of the LED was controlled by an LED controller (Zhongjiao Jinyuan Systems) and calibrated by a Laserpoint calibrator (A-02-D12-BBF).

Synthesis
All DASAs and its intermediates were synthesized according to a modified strategy based on the previous reports [8] .

Synthesis of DASA-2C
Scheme S1. Schematic illustration of the synthesis of DASA-2C.

Photoisomerization of DASAs in solutions
Due to the push-pull nature of DASAs, the absorption spectra shift in different solutions (Figure S1-S4

Photoisomerization of DASAs in the solid state
The isomerization of DASAs in the solid state was investigated by formation of films on the surface of glass substrates via spin-coating at 1000 rpm for 30 s ([DASAs]=0.01 M). A splitting and widening n-π* absorption band at ~540-650 nm was observed for DASA-2C in the solid state, which is attributed to the strong intermolecular π-π aggregation (Figure S8-S9). The n-π* absorption band gradually narrows with prolonging the carbon spacers for DASA-6C and DASA-12C, indicating the intermolecular π-π aggregation is inhibited (Figure S10-S13). On the other hand, the n-π* absorption band splits and broadens again by further prolonging the carbon spacers for DASA-18C, which might be attributed to the crystallization on surface (Figure S14-S15).

Mechanical properties of DASAs films
The thickness of the DASAs films were determined by AFM, and the information has been summarized in Figure S16 and Table S1. However, due to the poor filmforming property of DASA-2C, which generates plenty of fragments on surface, the thickness is therefore not provided. On the other hand, the films of DASA-6C, DASA-12C and DASA-18C exhibit similar thickness ranged between ~70 and ~100 nm.  where a is the contact radius of probe and sample, R is the tip radius, w is the adhesion energy, F is the loading force, δ is the indentation depth, * is the reduced Young's modulus. And , are Poisson's ratio and Young's modulus, respectively. Since ≫ , Force-distance curves of the DASAs films on silicon wafer substrates were summarized ( Figure S17-S20).

Fabrication of 2D logic-in-memory devices
The 2D logic-in-memory devices were fabricated via the following procedure: (1) Monolayer graphene/h-BN/sublayer graphene were picked up by polymer films composed of polycarbonate (PC)/poly-dimethylsiloxane (PDMS) and then deposited on silicon substrates.
(3) Electrodes were deposited on the heterostructure surface using the photomask lithography and metal thermal evaporation/lift-off process.

Light-controlling the 2D logic-in-memory devices
Depositing of DASAs on the surface of 2D devices shifts the Ids-Vg curves, indicating the interfacial effect and intermolecular interaction between graphene and DASAs ( Figure S22-S25). The 2D logic-in-memory devices exhibit negatively shifted Vg and gradually increased Ids upon irradiation while using DASA-6C, DASA-12C, and DASA-18C as the photoswitches (Figure S26-S27). As expected, the Ids-Vg curves of the devices with DASA-2C does not shift after irradiation due to the strong intermolecular π-π stacking ( Figure S29).