Discrete two-terminal single nanocluster quantum optoelectronic logic operations at room temperature

Abstract

Readily formed at nanoscale break junctions, arrays of individual spatially isolated, strongly electroluminescent Ag₂–Ag₈ nanoclusters perform complex logic operations within individual two-terminal nanoscale optoelectronics devices. Simultaneous electrical excitation of discrete room-temperature nanocluster energy levels directly yields AND, OR, NOT, XOR, and even full addition logic operations with either individual nanoclusters or nanocluster pairs as the active medium between only two electrodes. Imaged in parallel, noncontact electroluminescent readout obviates the need for electrically isolating individual features. This gated, pulsed, two-terminal device operation will likely drive future nano and molecular electronics advances without complicated nanofabrication.

Molecularly engineered electronics hold great promise for producing functional nanoscale devices (1–3), yet difficulties in wiring three independent electrodes in nanoscale dimensions continue to limit advances and even preclude measuring polarity in such devices (4–7). As the active elements shrink toward the molecular scale (8–10), standard transistor-based circuit size remains limited by electrode fabrication and addressability. Even with current nanodevices (11–15) room-temperature operation using discrete energy levels has not been demonstrated. Such room-temperature nanoscale quantum electronic devices would enable more complex calculations with vastly simpler circuits than possible with standard transistors (16–18). Here we report two-terminal optoelectronic logic operations using quantized single Ag nanocluster (19) energy levels at room temperature. Achieved through pulsed operation, the first pulse gates electron injection into discrete molecular/nanocluster energy levels by the second pulse. This pulsed, energetically discrete, two-terminal operation greatly simplifies nanoscale electronics device fabrication, yielding room temperature AND, OR, XOR, and NOT logic operations and even a full adder from individual parallelized single Ag_n (n = ≈2–8 atoms) nanoclusters (19) with inherent electroluminescent readout (20). The difficulties associated with electrically isolating similar or different single molecules are circumvented through optically reading the unique electrical response of each molecule.

Readily formed in both chemically prepared and radio frequency-sputtered thin Ag films, 1D arrays of nanoscale break junctions (21) are fabricated in situ by computer-controlled passage of voltage-limited current through the films for ≈30 s. This gentle electromigration process (21, 22) reproducibly forms arrays of spatially isolated electroluminescent Ag_n nanoclusters in the resistive break junction region. Formed Ag_n nanoclusters are likely to be closer to the anode because of the momentum transfer from the “electron wind” during electromigration (22). Voltages applied to the anode with the cathode as ground produce strong electroluminescence (EL) from current flow through multiple electrically written individual Ag nanoclusters spanning insulating silver oxide layers on silver electrodes (Fig. 1) (20). Knowledge of this device structure enables construction of a detailed energy-level diagram that governs the EL process (Fig. 2). Experimental (23–25) and theoretical energy levels for the electrode (Ag) (23), barrier oxide (Ag₂O) (24), and active species (Ag₂–Ag₈) (25–27) dictate the necessary voltages to inject electrons into Ag_n excited states through the Ag₂O conduction band. Because Ag_n nanocluster ground states are within the Ag₂O bandgap, injected holes are trapped by the oxide layer (Fig. 2). Independent of electrode separation, the entire applied voltage is dropped at the emissive junction. The narrow recombination zone leads to the high-frequency enhancement attainable in these discrete single-molecule elements (20).

Figure 1.

Microscopic images of an Ag nanocluster junction. (A) Scanning electron micrograph (Hitachi S-3500H, 15-keV electron energy) of a typical silver nanocluster junction. (B) Optical image (0.2-s charge-coupled device exposure collected with a 1.4 numerical aperture, ×100 oil-immersion objective) from the region bounded by the white box in A. White circles represent the EL areas corresponding to those further magnified in the scanning electron micrograph images (C and D). Formed in situ by dc excitation (20), ac-excited emission [163-MHz sine wave with 5-V_pp (±2.5 V) amplitude] arises from species too small to be observed with scanning electron micrography (<5 nm).(E) Degree of oxidation with distance from the junction as determined with energy dispersive spectroscopy (ThermoNoran, Middleton, WI, attached to Hitachi S-3500H) from 3-μm diameter regions. The poor energy dispersive spectroscopy spatial resolution was necessary to obtain sufficient signals, but it still clearly indicates a silver-rich junction surrounded by silver oxide between the pure silver electrodes. Numbers on the curve (Ag/O ratio) are the inverse of the y axis (O/Ag ratio) to more clearly indicate approximate Ag/O atomic ratios in different regions.

Figure 2.

Composite energy diagram governing silver nanocluster EL. The photoelectric work function of Ag(111) (4.74 eV) (23), x-ray and inverse photoelectron spectroscopy of the Ag₂O band structure (24), gas-phase ion- ization potential (5.66 eV) (25), and calculated electronic energy levels (26, 27) of Ag₃ all are plotted on the same energy scale to illustrate EL from silver trimer. Other small nanoclusters also have both visible emission and molecular energy levels favorable for EL in this device geometry (25). (A) This diagram shows that holes can be trapped in silver nanoclusters because the hole (h⁺) is located within the oxide band gap. After the hole is formed by field extraction, election reinjection causes either nonradiative thermal decay (B) or excited-state injection with subsequent emission (C) (20). Optimal hole formation and electron reinjection times are found to range between 2.8 and 3.5 ns from two-pulse excitation experiments.

To probe the time scales relevant to the EL process and perform logic operations, two consecutive high repetition rate narrow pulses (25 MHz) were used to initially inject holes then reinject electrons. EL intensities were recorded as functions of pulse width, polarity, amplitude, and interpulse delay. Contrary to dc studies, (20) pulsed excitation introduces polarity; EL is observed only for positive- followed by negative-pulse combinations. Emission is most enhanced when narrow positive and negative pulses (<3.5 ns, full width half maximum) are sequentially injected within ≈4 ns of each other. Although some variability in pulse widths arises in different devices (≈2.8–3.5 ns), interpulse delay is always limited by the two pulse widths and corresponds to the ac enhancement frequency. The first pulse establishes a field across the junction to inject holes into the nanoclusters by tunneling through the oxide layer, essentially concentrating charges in the nanocluster region for a short time. Because of the junction capacitance and the higher energy nanocluster excited states relative to the Ag electrode Fermi level, only when a negative second pulse follows a positive first pulse can the second pulse reach the nanocluster junction and directly reinject electrons into the EL-producing Ag_n nanocluster-excited electronic energy levels. Because the oxide barrier at the junction prevents direct electron reinjection into the nanocluster ground states, quenching of EL is avoided (Fig. 2). Obviating the need for a third electrode, the first short pulse acts as a gate by concentrating holes in and around the active molecules for a short time. Establishing a field to extract electrons, the first-pulse amplitude does not interact with discrete Ag nanocluster energy levels, but relates to a probability of hole injection in the active medium. Acting as a gate in this two-terminal device, increasing first-pulse amplitude increases the probability of hole injection in each individual molecule, thereby increasing total EL (Fig. 3).

Figure 3.

Quantum optoelectronic characteristics of typical Ag nanocluster junctions. (A) Typical Ag_n EL region with two consecutive pulse excitation (25-MHz repetition rate) exhibiting blinking (28–30) and dipole emission patterns (19, 20, 31) characteristic of individual molecules. The EL shows a strong pulse polarity dependence and is detected only with a positive followed by a negative pulse with proper widths (≈2–4 ns), amplitudes, and interpulse delay (<4 ns). Other two-pulse polarity combinations yield no detectable emission. The EL intensity also depends strongly on the amplitudes of the positive first and negative second pulses. (B) Integrated EL from ≈30 molecules exhibits nearly monotonically increasing emission with both first- and second-pulse amplitudes. Positive pulses 4 ns ahead of the negative pulses, each with 2.6-ns pulse widths and 1.0-ns transition times, were used. On/off ratios of 15 were obtained with −2.1-V signal pulses (the second pulse) after either 1.59-V (off) or 3.15-V (on) gate pulses. (Inset) The lowest voltage case, showing the threshold behavior, is expanded. (C) Integrated emission from five similar molecules exhibits three clear peaks along the second-pulse amplitude at −1.6, −2.1, and −2.85 V. EL increases monotonically with first-pulse (gate) amplitude, showing no discrete features except as a function of second-pulse amplitude. (Inset) Expansion of the lowest first-pulse amplitude trace to show the high contrast obtained and therefore the discrete nature of the individual molecular/nanocluster energy levels.

When applied within ≈4 ns of the positive first pulse (gate), the negative second pulse reinjects electrons into discrete Ag_n nanocluster-excited electronic energy levels. When properly applied, these pulse combinations enable current to flow with EL resulting from electron–hole recombination within each nanocluster. Intrinsically molecular species, different nanoclusters produce strong EL only when the second-pulse amplitude precisely corresponds to excited-state electronic energy levels. Because the first pulse is for hole injection, only the second pulse directly injects electrons into the discrete energy levels of the single Ag_n nanocluster junctions composing the recombination zone. Measurements of EL intensity (I_EL) from individual nanoclusters while adjusting the second-pulse amplitude from 0 to −3.15 V are thus expected to yield several peaks in the I_EL vs. voltage curves. However, because the number and position of peaks strongly depend on the electronic energy levels of individual nanoclusters, integration of the whole EL signal from all of the different nanoclusters in one device (>100) yields strong, first-pulse gated EL with two-terminal operation, but without observation of discrete levels as a function of second-pulse amplitude. Even with ≈30 molecules, the discrete nature of single molecule energy levels is blurred, yielding a nearly monotonic increase in EL with increasingly negative pulses after the positive gate pulse (Fig. 3B). Only the EL signals from individual nanoclusters or from groups of those with similar properties exhibit discrete nanocluster energy levels (Fig. 3C). EL from each individual nanocluster and that summed from similar nanoclusters definitively show that EL continuously increases with first-pulse amplitude (gain), but exhibits clearly resolved peaks with second-pulse amplitude (Fig. 3C). Thus, with direct EL readout from selected individual nanoclusters, this device can be used as a time- and voltage-gated, two-terminal quantized single molecular switch when operated with properly designed gate and input signal pulses into a single electrode. Because only two electrodes are necessary, even the simple analog gating with the first pulse makes these elements useful as single molecule devices without complicated nanofabrication of source, drain, and gate electrodes. The varied but discrete second-pulse voltage dependence of EL from different nanoclusters can be further used with parallel EL readout to perform significantly more complex operations within a single device than possible with standard transistor-based electronics.

This two-terminal, parallel, single-molecule excitation with extremely sensitive EL readout (Fig. 3A) results from electron injection into discrete energy levels and enables room-temperature circuit construction with much greater simplicity than possible with standard electronic components. Using the inherently parallelized collection of quantized EL features and the simple gated emission of multiple spatially isolated individual molecules, a variety of logic functions can be performed. Although examples of logic operations based on the simple gating behavior with the first pulse are not explicitly shown here, the fundamental data are given in Fig. 3. The high nanocluster emission on/off ratio is a strong function of first-pulse (gate) and second-pulse amplitudes and polarity, turning on only with specific second-pulse voltages. Provided that the positive first pulse for field-dependent electron extraction (hole injection) is always added at the prescribed time before the second pulse, the nonmonotonic I_EL-V characteristics enable combination of several input signal pulses as a composite second pulse to yield different logic gates. In this manner, AND, OR, NOT, and XOR gates, as well as complicated combinations of these logic gates, are readily produced from different molecules within individual two terminal devices (Fig. 4). Although the output is light and therefore difficult to propagate to subsequent devices, these optoelectronic devices are an important advance as they use discrete room-temperature energy levels to form true single molecule logic devices.

Figure 4.

Quantum optoelectronic logic operations with coupled individual silver nanoclusters at room temperature. (A) Using 2.6-ns pulses at 25-MHz repetition rates, a 2.2-V first pulse was always added before the second composite pulse, which itself was constructed from three separate input pulses (P₁, P₂, P₃), each with either 0.0- or −1.05-V amplitude. Voltages described below refer to the composite second-pulse amplitudes. (B) Nanocluster S (sum) is turned on (an output of 1) at −1.05 and −3.15 V and turned off (an output of 0) at −2.1 V. Nanocluster C (carry) is turned on at −2.1 and −3.15 V and turned off at −1.05 V. When only two input pulses with 0 V (an input of 0) or −1.05 V (an input of 1) amplitude are used, S and C act as XOR and AND gates, respectively to constitute a half adder. For example, if only P₂ and P₃ are used as inputs, C = [(P₂ OR P₃) AND NOT(P₂ AND P₃)] and S = (P₂ AND P₃). The on/off ratios for XOR and AND molecules are 6.0 and 12.2, respectively. The same molecules can also be fed with three inputs to work as a full adder. Input pulses and results for arithmetic addition of the corresponding three bits are shown (A). (C) The discrete EL-producing energy levels of these two nanoclusters enable operation as a full adder. Nanoclusters S and C act as the output nodes in the logic diagram, incorporating nine basic binary logic gates typically requiring at least 25 standard field effect transistors. In the full adder implementation demonstrated in A, nanocluster S = [(P₂ XOR P₃) XOR P₁] and nanocluster C = [P₁ AND (P₂ XOR P₃)] OR (P₂ AND P₃), in which XOR is defined as: a XOR b = [(a OR b) AND NOT (a AND b)].

All emissive nanoclusters in all such devices exhibit similar discrete injection behavior and can perform different logic operations. Differently designed input pulses can even yield different operations from the same molecule by being on or off resonance with EL-producing excited nanocluster energy levels. For example, one individual EL nanocluster (Fig. 4C, molecule S) was gated with a +2.2-V first pulse followed by different composite negative second pulses. Because this particular molecule produces EL when excited with a second pulse of −1.05 V, but not at −2.1 V, combinations of two signal pulses each either 0.0 or −1.05 V (i.e., 0 or 1, respectively) produce four possible composite second pulses with amplitudes of 0 V (one combination), −1.05 V (two combinations), or −2.1 V (one combination). Thus, only a 0 combined with a 1 in either channel yields EL. This nanocluster, therefore acts as an XOR gate with two 0- or −1.05-V input signal pulses overlapped to make one composite second pulse (Fig. 4, molecule S). Contrary to serial concatenation of multiple logic gates (typically using eight standard three-terminal field-effect transistors) to construct one XOR gate, the single XOR nanocluster between only two terminals successfully performs the parallel XOR operation. This same molecule also operates as a NOT gate with more complex pulse sequences. By repetitively applying a composite gate pulse consisting of a +2.2-V first pulse 4 ns ahead of a trailing −1.05-V pulse, single-molecule EL is continuously observed. Combination of this gate pulse with an additional −1.05-V signal pulse overlapping that incorporated with the composite gate pulse yields no EL. In the same manner, AND and OR gates can also be implemented with different nanoclusters and different gate and input signal pulse amplitudes corresponding to the proper energy levels for each individual molecule logic gate. More strikingly, by injecting electrons into excited states with energies more negative than (i.e., higher energy than) −2.1 V in the molecules shown in Fig. 4 so that three or more input pulses can be fed into the two-terminal integrated device, more complicated logic operations can also be performed. The discrete but coupled responses of two different Ag_n nanoclusters enable their EL to perform full addition operations. Using standard circuitry, a full adder would consist of nine basic binary logic gates (requiring at least 25 standard three-terminal field-effect transistors; ref. 18), but is successfully implemented with two single nanoclusters in a single two-terminal integrated device (Fig. 4A).

Whereas different nanoclusters have different energy levels, the I_EL vs. V_{2nd pulse} for each can be characterized and subsequently used to readily yield logic gates with more refined pulse input requirements. Thus, although formed in situ in the nanoscale break junction, this is a general method of using nanomaterials to form two-terminal quantum electronic logic gates with noncontact EL readout. Such pulsed operation with properly designed electrode systems greatly simplifies the construction of nanoscale electronics components. Electron injection into discrete electronic energy levels offered by these robust Ag nanomaterials at room temperature opens the way to vastly simplified circuit design with naturally parallel optoelectronic information processing. Because the active regions of most single-molecule and monolayer devices are extremely narrow, two-terminal device operation will be extremely important to future advances in nanoelectronics without complicated nanofabrication. The simplification of logic gates afforded through using the discrete room-temperature molecular energy levels of properly designed single molecule/monolayer devices is likely to fuel novel nanoelectronics advances.

Abbreviation

EL

electroluminescence