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Capacitive Effects in a Three-Terminal Organic Nano-Device

Capacitive Effects in a Three-Terminal Organic Nano-Device

Capacitive Effects in a Three-Terminal Organic Nano-DeviceReis et al.RESEARCH ARTICLEbe utilized as molecular gate; (c) the carrier accumulationis inversely dependent of length when conjugated rings areattached with molecular radicals; (d) occurrence of structuraldeformation as soliton, polaron or bipolaron in thebackbone of the structure.Recently, it raised up that molecular systems have competitiveeffects between diffusive and ballistic conductionand understanding these capacitive properties should beaddressed and controlled in nano-scale. 19 In the presentwork we investigate by quantum mechanics calculationsthe capacitive properties of organic three-terminal devices(Fig.1).In the next section, the system and methodologyutilized and the results and conclusions will be presentedin the last two sections.2. SYSTEM AND METHODOLOGYWe investigate a 3-terminal device with the intention toreach out new transport properties unobserved in macrodevices or two-terminal nano-devices.This signature couldbe followed by the quantum nature of organic structure asa single electronic compound and as Non-ohmic behaviorfor molecular electronic devices.It has been recentlyaddressed 20 showing nonlinear voltage–current characteristicas an intrinsic organic thyristor.The molecular structureof controlled molecular rectifier (CMR) is showed inFigure 1 (Left).Hartree-Fock (HF) approaches contained in Gaussianpackage 21 was employed including standard basis setswere used for all calculations performing the same qualitativeresults.The geometric forms of the analyzed structureswere fully optimized using HF methodology including aVoltage as an external electrical field in form of Roothaan-Hall matrix in a model within closed shell.The methodology presented here is based on the same22 23grounds used in successful model presented elsewhere.This is a general procedure to study nanostructuredsystems and it is a competitive technique compared with24 25other ones utilized in materials engineering designsuch as that involving the quantum transportation.This isthe first theoretical simulation of capacitive effects in anidealized organic 3-terminal prototype nanodevice.Also,we would like to stress that this procedure is not dependingon specific parameters or empirical values.It is basedon first principle quantum mechanics calculations.3. RESULTS AND DISCUSSIONFundamentally, usual devices can have two capacitiveeffects for high frequencies: depletion (or transition) anddiffusion (or accumulation) capacitance.Under reverse andforward polarization occur the transition (Ct) and diffusion(Cd) capacitances, respectively.When the device is undera reverse polarization, the depletion region increases withthe reverse potential and decreases the transition capacitance.Inthe opposite way, for forward polarization, thecapacitance is dependent of injected carriers for the outsidedepletion regions, e.g., it depends on main carriers ofthe device.Also, high current levels have a dependence ofhigh diffusion capacitance level.Henceforth, in the molecular level the capacitive effectsare different but the concept remains correct and it has thevalidation as a way to describe the charge in the interfacemolecule-electrode.Iafrete et al. 26 propose in molecularlevel the capacitance as:e 2CN =(3)IPN− EAN for a closed shell system with N -electrons, where the ionizationpotential (IPN) and the electro-affinity (EAN )should be take into account.The molecular capacitance is a function of number ofelectrons in the system, in opposite way of usual capacitorsthat depends of its own liquid charge distribution over thedevice shape.Fig. 1. Pictogram of Controlled Molecular Rectifier (CMR): (Left) Horizontally the device presenting the gate molecular structure composed by aconjugated polymer separated by aliphatic CH 2 atoms and almost vertically (connected to +Vg) the main rectifier molecule within donor and acceptorgroups attached in the extremities; (Right) Pictorial analogy with usual devices including p–n semiconductor junctions and depletion regions.2 J. Comput. Theor. Nanosci. 6, 1–5, 2008

Reis et al.Capacitive Effects in a Three-Terminal Organic Nano-DeviceIn Figure 1, it presents a pictogram representing themolecular structure calculated in this work as well as asemiconductor model presenting p–n junctions.The calculatednanodevice obtained (left) was represented by pand n regions as usual semiconductor devices do.Followingthe majority carrier it is possible to identify the classof clusters as well as the depletion region that permitsthe device works as semiconductor device (right).Thesedepletion regions found in the molecular gate composedby CH 2 aliphatic groups and they are responsible for thequantum wells in the system.However, for a +V g , themajority carriers by drift current provoke the p regiondoping increasing the number of carriers (electrons), anddepending of direction of applied voltage, it will appearcharge diffusion in the same direction of polarization.In Figure 2, it presents the capacitance versus voltagefor the molecular gate of the device.Within the increase ofreverse polarization, the charge accumulation in the gateabruptly decreases due the increase of molecular quantumwell, e.g., increasing of the depletion region. We observethat appears a transition capacitance by charge accumulationin the lowest unoccupied molecular orbitals whiledecrease under reverse bias.However, for forward biasappears tunneling of majority carriers through the quantumwells, which are undesirable for any capacitor.The variationof transition capacitance at reverse bias can be utilizedas a usual varactor diode (variable voltage capacitor).In Figure 3, it presents the capacitance versus voltagefor the main molecule of the device.The main moleculeis in the off state between −3.5 V and 2.0 V and it mainlypresents depletion capacitance by majority carriers fromthe -orbitals in the extremities of molecular terminals (T 1and T 2 ).In the on state (under forward and reverse bias)presents charge increase by diffusion in the main moleculeimplying in high diffusion capacitance.Fig. 3. Capacitance–Voltage for positive applied bias in T 1 –T 2 terminalsrepresented in Figure 1.In Figure 4, it presents the theoretical absorption spectraby HF methodology including configuration interactionto give the best description of the UV-Visible-IR opticaltransitions and taken into account from the first 12 unoccupiedmolecular orbitals to the last 12 occupied molecularorbitals, including singlet states.Figure 4(a) shows threemain bands centered at 188 nm, 258 nm and 337 nm composedby transitions presented in the figure.The molecularsystem under a reverse {Fig.4(b), see the representation of−3.5 V in Fig.3} and forward {Fig.4(c), see the representationof 1.8 V in Fig. 3} bias rose up a very important pattern:When the voltage goes to the operational on switch,a strong red shift of main transitions appears composedRESEARCH ARTICLEFig. 2. Capacitance–Voltage for the CMR investigated applying anexternal electrical field in gate terminal (Represented by V g in Fig.1).Fig. 4. Theoretical absorption spectra of CMR device for (a) 0.0 V,(b) −3.5 V and (c) 1.8 V applied bias. The H and L means the Highestoccupied molecular orbital and the Lowest unoccupied molecular orbital,respectively.The AH−x → L+y is a representation of a HOMO minusx to LUMO plus y molecular transition weighted by the A coefficient ofLCAO (linear combination of atomic orbitals) expansion.J. Comput. Theor. Nanosci. 6, 1–5, 2008 3

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