The senses are the basis of human interaction with the external environment. Among them, olfaction is one of the earliest developed senses in humans and is closely intertwined with cognitive analysis in the brain. Within the human olfactory system, olfactory neurons are activated by various odor molecules, initiating a series of synaptic events^[1−6]. Abnormalities in the olfactory system may be an early indication of brain neurodegeneration or disease. Current research on the olfactory system in biomedicine can be applied to develop new methods for treating neurological diseases or to create bio-hybrid devices that surpass natural sensory capabilities^[7−11]. For instance, Zhang et al. have integrated DNA origami and DNA scissor techniques to achieve an unprecedented level of sensitivity, accuracy, and practicality in surface plasmon resonance (SPR) biosensors^[12]. They also developed the methodologies of photonic CRISPR sensing (MOPCS) gene detection platform for highly sensitive analysis of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)^[13]. Shen et al. have developed an Sr-ZrO two-terminal gas sensor that can identify and remember while providing self-protection against ammonia gas^[14]. Ding et al. demonstrated a gas synaptic diode based on pentacene/amorphous IGZO that integrates sensing, processing, and storage functions into a single device for monitoring ammonia gas^[15]. With a high demand for environmental safety and medical applications, emulating the synaptic behavior between olfactory neurons holds significant importance for the future construction of neuromorphic computing and biomimetic sensory systems^[16−18]. Compared to the majority of reported two-terminal memristor olfactory synaptic devices, three-terminal transistor synaptic devices are capable of performing signal transmission and learning functions concurrently. With the aid of gate voltage, these devices can achieve precise control over the synaptic modulation, thereby avoiding crosstalk issues between neighboring units^[2]. However, traditional transistor-based gas sensors merely convert gas concentration into electrical signals, lacking the memory capabilities akin to synapses^{[19, 20]}. To solve this problem, we propose a flexible artificial chemosensory neuronal synapse, where the synaptic properties of the transistor are induced by the energy band barrier between organic heterojunctions^{[21, 22]}. Transistors can transmit signals through the channel layer after receiving and reading stimuli, modulate synaptic weights via the back-gate electrode independently, this device efficiently integrates the functions of data acquisition, transmission, and storage, overcoming the drawbacks of separate gas sensors and synaptic diodes, thereby significantly reducing the complexity of artificial olfactory neural systems.

We focus our work on the perception of ammonia gas. As a weak reducing and harmful gas, ammonia can corrode human skin, eyes, and lungs^{[23, 24]}. Inhaling high concentrations of ammonia can cause chemical inflammation of the upper respiratory mucosa and even damage the central nervous system^{[25, 26]}. Ammonia gas sensors are utilized in a variety of fields including environmental monitoring, agriculture, counter-terrorism, biomedicine, and industrial waste management^[27]. However, traditional ammonia gas sensors based on polymer materials and oxide semiconductors often suffer from limitations such as high cost, high power consumption, large size, and demanding maintenance requirements. As an alternative, organic material-based gas-sensitive materials have garnered widespread attention due to their high sensitivity, lightweight flexibility, molecular design diversity, and unique partial biodegradability^[28−30]. Here, pentacene and C8-BTBT, as small molecule organic semiconductor materials, are deposited by vacuum thermal evaporation. Their semiconducting π-conjugated systems are amenable to modification by external chemical signals. By integrating with multifunctional biological systems, these materials can endow organic sensors with the capability of sensory perception^[31]. Our device is employed to expose the active layer C8-BTBT to the atmosphere with the structure of bottom gate top contact (BGTC), enabling a rapid response to ammonia at ppb levels. Additionally, under the regulation of hole transport in the organic semiconductor heterojunction and gate voltage modulation, a 15 V storage window emerges due to the non-volatile storage characteristics of the device. The interaction between ammonia molecules and the p-type organic semiconductor film enables gas-sensitive responses akin to information storage between neurons. Upon stimulation by NH₃ pulses, a series of inhibitory synaptic behaviors are mimicked, including inhibitory postsynaptic currents (IPSC), paired-pulse depression (PPD), transition from short-term depression (STD) to long-term depression (LTD), and long-term potentiation/depression (LTP/LTD). The olfactory synapse transistor (OST) can still detect synaptic weight changes triggered by NH₃ even under bending conditions, thus reflecting the portability and bionic application value of this synaptic device. This study will further contribute to the olfactory recognition of gases such as NO₂, SO₂, or specific volatile organic compounds (VOCs)^[32].

Firstly, the flexible PI is used as the substrate, and 100 nm thick indium tin oxide (ITO) is deposited as the bottom gate electrode using physical vapor deposition (PVD) at a pressure of 2 × 10⁻⁶ mbar. Afterwards, poly(propylene carbonate) (PPC) and poly(melamine-co-formaldehyde) (PMF) are dissolved in propylene glycol methyl ether acetate (PGMEA) with a mass ratio of 2 : 1 in a 10 ml solution. The mixture is stirred magnetically at 90 °C and 1000 rpm for 24 h to form a transparent and uniform solution. The blended PPC solution is then spin-coated onto the PI/ITO substrate at 2500 rpm for 60 s. To ensure good contact between the PPC film and the PI/ITO substrate, the PPC film is annealed at 130 °C for 1 h in a drying oven^[33]. Subsequently, the stack of pentacene/C8-BTBT with 50/20 nm thickness is deposited by vacuum thermal evaporation at a pressure of 4 × 10⁻⁶ mbar and a deposition rate of 0.5 Å/s. Finally, 70 nm thick Au is deposited as the source−drain electrodes using PVD through a shadow mask, as shown in Fig. 1(a) illustrating the structure of the OST.

The cross-sectional view of the OST is observed using scanning electron microscopy (SEM), as shown in Fig. 1(b), revealing the thickness of the degradable PPC film as an insulating layer is about 1 μm and the sharp interfaces between organic layers.

Fig. 1(c) displays the transfer characteristics of three different flexible organic transistors (V_DS = −15 V), with the gate voltage (V_G) scanned bidirectionally within the range of −40 to 40 V. All devices exhibit normal p-type operating mode behavior. The individual C8-BTBT OFET demonstrates a low drain current (I_DS), while the maximum threshold voltage (V_TH) of the individual pentacene OFET is 35 V. The V_TH of the organic heterojunction transistor is 10 V, with a current on/off ratio reaching 10⁶. The continuous C8-BTBT film deposited on top of the pentacene acts as an interfacial layer that injects hole carriers from the source/drain electrodes into the pentacene, and it also provides an additional hole transport channel. Due to the energy band bending caused by the heterojunction, the interface region can induce electrons and holes on both sides, leading to the filling of localized states. Under the action of the V_G, the hole transport channel at the interface is opened, and the dual-channel transport caused by the heterojunction structure results in a shift of V_TH. Moreover, compared with other single organic semiconductor layer transistors, the olfactory synapse transistor (OST) has a significant hysteresis window. The counterclockwise hysteresis window approaching 15 V being driven by energy band regulation between the organic heterojunctions. Due to the smaller bandgap of pentacene (2.1 eV) compared to C8-BTBT (3.7 eV), an effective carrier channel is expected to be formed in the pentacene layer^{[10, 34]}. The holes transferred from the C8-BTBT layer can cause an asymmetric carrier distribution in the pentacene and consequently stay in the highest occupied molecular orbital (HOMO) band of the pentacene for a long time with fewer recombination. Fig. 1(d) illustrates the output characteristics of the OST, demonstrating a well-maintained linear relationship between V_DS and I_DS while V_G is controlled from −10 to −35 in step of −5 V.

It is well known that both C8-BTBT and pentacene are p-type semiconductors, and the lone pair of electrons present in polar ammonia molecules manifest as donator-like trap states within the p-type organic materials^[15]. Due to the increase in energetic disorder from charge-dipole interactions, ammonia molecules diminish the hole transport in organic channels. During the testing process, the flow rate of the NH₃ pulse is uniformly set to 0.33 ml/s. As depicted in Fig. 2(a), the response of three different organic transistors to a NH₃ pulse (100 ppm, 10 s) is illustrated. Response is computed as (I₀ − I_t)/I₀ × 100%, where I₀ and I_t represent the current values exposed to air and under the current NH₃ concentration, respectively^[35]. Under the effect of a NH₃ pulse (100 ppm, 10 s), all three organic transistors exhibit significant current drift phenomena, with response gradually increasing over time, indicative of continuous NH₃ molecular adsorption. Compared to transistors with a single organic layer, the OST demonstrates a stronger response, attributed to the deposition of the C8-BTBT film on pentacene with more suitable grain sizes, significantly enhancing the adsorption capacity of NH₃ molecules. Fig. 2(b) presents the transfer characteristics of the OST under different NH₃ concentrations, where higher NH₃ concentrations offer more electrons within the characteristic time to deplete some holes in the channel, resulting in decreased on-state current of the device under the same V_G and leftward shift of the threshold voltage (V_TH). Fig. 2(c) showcases the response of the OPT to different concentrations of NH₃, with the device achieving a response of 5% for 100 ppb NH₃. Fig. 2(d) illustrates the IPSC of the OPT to a NH₃ pulse (100 ppm, 10 s) under different gate voltages, with the device response current being at pA levels at the V_G of +20 V. As the V_G sweeps from +20 V to 0 V and then to −20 V, the increase in carrier concentration in the channel of device leads to a higher charge exchange efficiency, resulting in a gradual increase in the device response current. In addition, the organic heterojunction channel with a properly aligned band structure not only considerably improved the NH₃ response of the synaptic device, but also enabled the implementation of the most essential feature for non-volatile characteristics.

Fig. 3(a) illustrates the working mechanism of olfactory neurons. When an action potential is generated in the presynaptic neuron and transmitted to the postsynaptic membrane, chemical synapses release neurotransmitters by opening ion channels, thereby activating the synapse. As an artificial olfactory neuron, the V_DS is always maintained at 10 V in the process of receiving the NH₃ pulse. The OST exhibits typical PPD behavior. The inset in Fig. 3(b) depicts the current response triggered by two consecutive NH₃ pulses with a time interval Δt of 2 s and pulse width Δτ of 5 s. The current peak (A₂) under the second NH₃ pulse is 1.3 times higher than the current peak (A₁) under the first NH₃ pulse, indicating the superposition degradation and incomplete desorption of current caused by NH₃ molecular accumulation in the organic film. The PPD index, as shown in Fig. 3(b), is defined as the ratio of A₂ to A₁, which decreases with an increase of the time interval between the double pulses^{[36, 37]}. The experimental results are fitted to a double exponential decay relationship, and the model is as follows:

1PPD=A+B1exp(−Δtτ1)+B2exp(−Δtτ2).

Here Δt represents the time interval between two NH₃ pulses, B₁ and B₂ are the original facilitation magnitudes of the two phases, and τ₁ and τ₂ are the characteristic relaxation times of the two pulses respectively. The decay phenomenon illustrated in the double exponential fitting results aligns with the PPD behavior observed in synaptic neuros.

Furthermore, the impact of NH₃ stimulation in different modes on synaptic weights in neurons was also investigated, achieving the transition from STD to LTD^{[38, 39]}. Fig. 3(c) shows the IPSC of the device at different NH₃ pulse widths, where an increased duration of NH₃ leads to a higher increment in inhibitory current. Similarly, Fig. 3(d) exhibits a similar trend where the number of NH₃ pulses (100 ppm, 2 s) with a time interval Δt of 2 s continuously increases, resulting in a stronger current suppression effect. This observation aligns with the principles of repetitive learning and memory of gas information within the human brain. In the human olfactory neural system, synapses can selectively respond to high-frequency/low-frequency signals, thereby achieving a filtering function for gas information. Fig. 3(e) demonstrates that higher NH₃ pulse frequencies can induce a stronger current suppression effect, resembling the synaptic vesicle release, screening, and transmission of gas information in response to high-frequency signal stimulation. After the NH₃ pulses are removed, the sustained and stable inhibitory current indicates the long-term memory capability of the OST for gas information, implying a reduced dependence on activity in neuronal synaptic efficacy over an extended duration. The LTP/LTD processes, as depicted in Fig. 3(f), involve the application of 20 consecutive gate voltage pulses (10 V, 2.5 s) followed by 20 consecutive NH₃ pulses (100 ppm, 2.5 s) to the OST, while monitoring the postsynaptic current (PSC) with a read bias of 10 V. At equilibrium (V_G = 0), a portion of the electrons reside in the pentacene layer near the dielectric layer interface, while hole carriers accumulate slightly in the vicinity of the dielectric layer. Positive electric pulses cause a sustained increase in the PSC, which may be due to the capture of electrons at the pentacene/dielectric interface, enhancing the hole injection rate from the presynaptic terminal to the channel. This results in an increased concentration of hole carriers at the organic heterojunction interface and maintains carrier asymmetry in the channel region. When the device is stimulated by continuous NH₃ pulses, due to the smaller bandgap of pentacene compared to C8-BTBT, a large number of lone pair electrons transfer to the organic heterojunction interface and rapidly recombine with the hole carriers in the HOMO level of pentacene, disrupting the previously asymmetric carrier distribution in the channel region and restoring the PSC to its previous current level. The persistent change in PSC indicates that the OST has continuous modulation behavior, effectively simulating the synaptic weight modulation observed in biological synapses.

In order to characterize the flexible performance, the OST was tested in various bent states. Fig. 4(a) illustrates the schematic diagram of the bending test, where the highly transparent PI substrate with a thickness of approximately 0.05 mm is bent on the surface of a semicylinder with a radius (R) of 5 mm, the maximum strain can be calculated to be ε_max = 0.5%^[40]. Fig. 4(b) displays the transfer characteristics of the device under different bending radius, indicating that the current on/off ratio remains consistent with the initial state when the device is bent at radius of 5, 10, and 15 mm. The slight degradation of I_DS can be neglected, suggesting both the transistor dielectric layer and channel material exhibit excellent flexibility. The NH₃ sensing characteristics of the device were also tested, as shown in Fig. 4(c), where OSTs with different bending radius demonstrated a high responsiveness to NH₃ pulses (100 ppm, 10 s), and the current level remained stable after the pulse ended. Lastly, the synaptic properties of the device under continuous modulation in bent states observed in Fig. 4(d). By applying 20 consecutive gate voltage pulses and 20 consecutive NH₃ pulses to the device, successful LTP/LTD processes were observed in the bent device. From the aforementioned analysis, our flexible synaptic device maintains stable electrical characteristics and olfactory synaptic features, even when subjected to smaller bending radius, while retaining transparency, thinness, and good flexibility.

In this work, a flexible olfactory neuron transistor based on organic heterojunction has been successfully fabricated. This device integrates NH₃ detection with human olfactory neuron mechanisms, which has been tested for responses to stimuli under different concentration conditions, and effectively mimics typical olfactory synaptic behaviors such as IPSC, PPD, transition from STD to LTD and LTP/LTD. It is worth noting that this device can maintain stable electrical performance and target gas sensing under various bending conditions, while also exhibiting synaptic behaviors under the modulation of electrical and NH₃ pulses. This opens up possibilities for the future application of biomimetic olfactory electronic devices.