Controlled Charge Trapping and Retention in Large-Area

May 4, 2016 - Here, we report on charge-retention transistors based on novel protein-mediated Au nanoparticle (NP) arrays, with precise control over ...
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Controlled Charge Trapping and Retention in Large-Area Monodisperse Protein Metal-Nanoparticle Conjugates Chang-Hyun Kim,†,‡,§ Ghibom Bhak,†,⊥ Junghee Lee,†,⊥ Sujin Sung,‡ Sungjun Park,‡ Seung R. Paik,*,⊥ and Myung-Han Yoon*,‡ ‡

School of Materials Science and Engineering and §Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea ⊥ School of Chemical and Biological Engineering, Institute of Chemical Processes, College of Engineering, Seoul National University, Seoul 08826, Republic of Korea S Supporting Information *

ABSTRACT: Here, we report on charge-retention transistors based on novel protein-mediated Au nanoparticle (NP) arrays, with precise control over dimension and distribution. Individual NPs are coated with alpha-synuclein, an amyloidogenic protein responsible for Lewy body formation in Parkinson’s disease. Subsequently, a monolayer of protein-NP conjugates is successfully created via a simple and scalable solution deposition to function as distributed nanoscale capacitors. Controllability over the film structure translates into the tunability of the electrical performance; pentacene-based organic transistors feature widely varying programmability and relaxation dynamics, providing versatility for various unconventional memory applications. KEYWORDS: electrical memories, organic transistors, protein self-assembly, charge trapping, metal nanoparticles rganic field-effect transistors (OFETs) with electrical memory functionality have developed significant momentum as an emerging technology for various future applications.1 The hysteresis observed in a dual-sweep measurement of their transfer characteristics, i.e., gate voltage (VG) versus drain current (ID), is a manifestation of the charge trapping/detrapping dynamics, and such a behavior can be reinforced by employing materials and/or geometries that temporarily immobilize charge carriers.2 Representative conventional concepts comprise floating-gate electrodes and ferroelectric dielectrics, which make use of tunneling barriers and polarization-switchable molecular orientation, respectively.3,4 More recently, metal nanoparticle (NP)-based OFET memories, suitable for scaling-down and integrated optical, electrical, and biological functionalities, have attracted significant attention.5 Baeg and co-workers demonstrated a controllable threshold voltage (VT) of organic nano floating-gate memory (NFGM) by fabricating Au or Ag NPs from thermal evaporation and placing them between tunneling and blocking gate insulators.6 The devices exhibited programmable/erasable charge storage inside the NPs with a highly nonvolatile character (retention time exceeding ∼104 s), and both p- and ntype behaviors have been successfully demonstrated.6,7 A different system, namely “NPs on dielectric”, was demonstrated by Novembre and co-workers. In their initial report, solutiondispersed Au NPs were attached to the amine-terminated selfassembled monolayer that passivates SiO2.8 A vacuumevaporated pentacene film directly contacted the NP clusters, creating hole-transporting memory OFETs with rather leaky

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behaviors (retention time in the order of 102 to 103 s). In later studies, the authors have extensively proposed synaptic computation, which directly benefits from the volatile charge storage and the corresponding rate-coding capabilities.9,10 Despite the great potential of NP-based organic memories proven in these studies, it is still desired to improve device fabrication methods and final device geometries. For instance, NPs from a thin metallic film are formed via aggregation governed by random diffusion kinetics, and thus it is difficult to control their sizes and distribution in a precise manner.6 Solution-based deposition also shows the limitation in terms of uniform dispersion although the particle size can be predetermined via synthetic route. Furthermore, specific chemical treatment of NPs and/or substrate is often required for adhesion.8 More importantly, no single technology has successfully demonstrated comprehensive modulation of charge retention time, from that usually obtained by the NFGM to that from the direct NP-semiconductor contact-based devices. We also note that the former often necessitates the fine optimization of the properties of two constituent dielectrics, and the latter can suffer from a semimetallic channel in the high-density NP regime.9 In this article, we report on a functional Au NP monolayer prepared via protein-mediated adsorption and demonstrate its charge-trapping and retention characteristics as an integral part Received: February 23, 2016 Accepted: May 4, 2016

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DOI: 10.1021/acsami.6b02268 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic representation of the αS−Au NP monolayer formation on SiO2. Specific interaction of αS (blue) with Au NPs (red) produces the homogeneous conjugates of αS−Au NPs (upper row). Buffer change from pH 6.5 to 4.5 induces adsorption onto SiO2, leading to the closely packed monolayer (lower row). (b) Structure of the OFET devices. (c) Optical images of water contact angles on bare and conjugatemodified SiO2. (d) FE-SEM images of αS−Au NP monolayers made of 5-, 10-, and 30-nm NPs (the particle size corresponds to its diameter). (e) Three-dimensional AFM images and thickness profiles of αS−Au NP monolayers made of 5-nm (top), 10-nm (middle), and 30-nm (bottom) NPs.

of OFET memories. Alpha-synuclein (αS) molecules induce strong binding of Au NPs onto SiO2, creating tightly packed conjugates with a uniform distribution and controllable dimensions via simple solution deposition. With the proteinNP monolayer incorporated into pentacene OFETs, an exceptionally high degree of device tunability is achieved with a correlation to the NP geometry. It was previously demonstrated that αS, an amyloidogenic protein with intrinsically disordered structure, can conjugate with Au NPs with high binding affinity,11 and such conjugates can be adsorbed onto glass or polycarbonate substrates at pH 4.5 in the form of largearea high-density single layers.12,13 This selective interaction of αS may result from its amino acid sequence and structurally disordered state, however, the exact mechanism needs to be investigated further. Recent studies also reported that αS could bind to citrate-capped NPs via N-terminal regions.11 This Nterminus-dependent binding to Au NPs could expose the acidic C-terminus of the unfolded protein on the surface of αS−Au NP conjugates. The structurally disordered and flexible state of the exposed C-terminus would be primarily responsible for the interaction between αS and the substrate. Given that the structural flexibility of intrinsically disordered proteins is capable of exhibiting versatile interactions with diverse biological partners, the multiple-interaction behavior of αS can be attributed to the structural plasticity of naturally unfolded αS structure. At an acidic pH of 4.5, therefore, the structurally flexible nature of the C-terminus enables αS−Au NPs to bind to the substrate, and the acidic nature of Cterminus lead to the repulsion among the αS−Au NP conjugates. This results in the formation of the conjugate

single layers without agglomeration. To our knowledge, this is the first article employing αS-conjugated NPs for organic electronic devices, although there exist several reports of preparing devices with other biomolecules. For instance, pentacene-based capacitors and transistors using streptavidin,14,15 aptamer,16 and DNA-hybridized17 Au NPs were reported, and they benefited from material-specific binding to target substrates. A closely packed Au NP monolayer was successfully formed by the pH-dependent adsorption of αS−Au NP conjugates on a SiO2 surface (Figure 1a and the experimental details in the Supporting Information). The basic properties of these conjugates and the αS-mediated binding of Au NPs were reported in our previous study.12 In brief, the αS−Au NP conjugates were prepared by encapsulating Au NPs with αS molecules during their incubation in 20 mM MES (pH 6.5) for 12 h at 4 °C. αS−Au NP conjugate formation was confirmed with dynamic light scattering (DLS) analyses (Figure S1). It revealed that the average hydrodynamic diameter of 10-nm Au NP coated with αS increased from 9.0 to 26.0 nm, indicating that the hydrodynamic thickness of the protein layer is estimated to be approximately 8.5 nm. After the buffer solution was switched to 50 mM citrate at pH 4.5, the αS−Au NPs were readily adsorbed onto SiO2 to form a homogeneous Au NP monolayer. Adsorption was conducted in a humid chamber at 40 °C to prevent evaporation of the buffer solution and obtain optimal packing density of αS−Au NPs. Subsequently, the substrate was thoroughly rinsed with 20% MeOH to remove unbound Au NPs and completely dried under N2, leading to a uniformly dispersed αS−Au NP monolayer. The adsorption B

DOI: 10.1021/acsami.6b02268 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces and workup procedures were optimized to obtain a homogeneous film. Washing with distilled water or adsorption at higher temperatures resulted in nonuniform layer structures or a substantial agglomeration of NPs, respectively (Figures S2 and S3). To finalize the device fabrication, we deposited pentacene, a high-performance organic semiconductor, followed by Au source/drain contacts onto the αS−Au NP layer (Figure 1b). The packing densities of the αS−Au NPs were analyzed with a field-emission scanning electron microscope (FE-SEM) (Figure 1d and Figure S4). The average densities of Au NPs with a diameter of 5, 10, and 30 nm were obtained as 2518.3 ± 153.3, 1365 ± 50.3, and 554 ± 21.7 μm−2, respectively. The packing density was controlled by adjusting the concentration of αS−Au NP solutions. Upon diluting the solution (30-nm Au NPs) 8 times, for instance, the packing density decreased to 104.7 ± 13.8 μm−2 (Figure 1d). The height profile of the αS−Au NP layers was assessed with atomic force microscopy (AFM). As shown in Figure 1e, uniform monolayer packing was confirmed, as the thickness measured over a few-μm length is comparable to the sizes of constituent particles. The contact angle images in Figure 1c are a macroscopic evidence for the chemical modification of hydrophilic SiO2 with αS−Au NPs. X-ray photoelectron spectroscopy (XPS) was also performed on αS−Au NP single-layered SiO2, and it verified the adsorption of NPs and proteins (Figure S5). The electrical function of an NP film was investigated by acquiring the transfer characteristics of OFETs. As shown in Figure 2a, a sweep-direction dependent behavior was observed in all cases, with its voltage-window significantly amplified when NPs are present. The small hysteresis in those without NPs can be attributed to the electron-trapping hydroxyl groups on SiO2 and/or oxygen-induced traps in pentacene.18−20 On the contrary, the NP-containing devices featured enlarged hysteresis in both the transfer and output characteristics (ID versus drain voltage (VD), Figure S6). This result indicates that NPs surrounded by protein molecules serve as additional chargetrapping centers, through which storing/discharging actions can take place under the time scale of sweeping measurements. Especially, the effect of particle density is clearly visualized on the output curves (Figure S6); whereas the low-density 30-nm NP sample had minimal hysteresis, the high-density one showed apparent kink-like features upon forward sweep, followed by much smaller reverse currents. The observed negative differential resistances are due to the strong chargetrapping capability of the high-density NP structure, due to which the NPs become already filled at the initial stage of a sweep, thus reducing the currents sharply afterward. As compared to the control device, the reduction in both onstate current and field-effect mobility was observed in devices containing NPs. A rough correlation can be made from the AFM images in Figure 2a that show smaller polycrystalline grains due to the increased roughness of the NP-coated substrate.9,21 Considering that the present device shows charge retention with a highly nonvolatile character, direct charge transport through an NP-to-NP pathway may marginally contribute to the apparent current, because the tunneling through and/or diffusion over the energetic barrier of insulating αS may not be significant at a weak lateral electric field in the given device geometry. Devices within each 1.5 × 1.5 cm2 substrate showed a narrow statistical parameter distribution (Figure S7). Therefore, a distinctive merit of our technique is the uniform packing of the NPs over large-area substrates

Figure 2. (a) Dual-sweep transfer curves measured in the saturation regime with different αS−Au NP structures. Corresponding AFM images of the 50-nm thick pentacene channel are placed below. (b) Symbols: gate-sweep range dependent memory windows. Lines: linear regressions. (c) Slope parameters extracted from the regressions in b.

(Figure 1d) and, thereby, a low fluctuation in on-chip device performances. The memory window, defined as the difference between the high and low VG values giving rise to the ID of 10 nA, is found to be quasi-linearly dependent on the VG sweep range (Figures S8 and 2b). This observation signifies the efficient injection and removal of charge carriers by VG, and such manipulation was characterized by the slope parameters in Figure 2c. To investigate the electrical role of αS encapsulation, we also conducted a simple comparison experiment. After creating an αS−Au NP monolayer (with 5-nm NPs) on SiO2, the substrate was exposed to prolonged plasma activation prior to the pentacene deposition, to selectively etch organic substances (protein) bound to metallic NPs. We found that, because of the direct contact of a semiconductor channel with metallic NPs, the transistor becomes more conductive in the both on- and off-states with smaller hysteresis, as compared with the device with an intact αS−Au NP monolayer (Figure S9). This result shows that αS functions as an active electrical component as well as a binder/monolayer-former. To evaluate the gate controllability, we proceeded with separating the programming and reading functions of VG through pulse-based electrical measurements. The transfer characteristics were extracted separately in the initial, programmed, and erased states using the same OFET; the devices were set with a fixed VG pulse of different polarities and magnitudes for 10 s (VD was kept at zero during the pulses). The transfer curves moved laterally with their overall shape practically unchanged, indicating that the pulse stimulus only C

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Figure 3. (a) Upper panel indicates the polarity and magnitude of the gate programming/erasing pulse applied between transfer measurements. Lower panel corresponds to ΔVT observed upon repeated measurements of the transfer characteristics at VD = −20 V. (b) Schematic explanation of the gate-bias programming and charge retention. Ef denotes the Fermi level of the electrodes and NPs. The flat-band potential is assumed to be zero for this diagram. (c) Maximum threshold voltage shift (ΔVT,max) and charge stored per area (Qmax). (d) Particle density (N) and number of holes per particle (S) extracted for memory devices. Insets: Illustration of the effect of NP distribution on interparticle electrostatic repulsion and charge occupation.

shifts the VT, whereas other device properties remain unaffected (Figure S10).6 The measurements were extended by applying alternating and growing negative and positive VG pulses from ±10 V to ±60 V and recording the transfer curves immediately after each pulse application (Figure S11). The threshold voltage shift (ΔVT) in Figure 3a visualizes control over the electrical state by VG in our memory devices and also provides an indication of their operation mode (ΔVT is in reference to the V T of the initial state). Under the first negative V G (programming) pulse, the thin protein encapsulation serves as a tunneling barrier (Figure 3b), and a part of accumulated holes can be trapped and energetically confined within NPs. Once this pulse is removed, αS now functions as a blocking dielectric (Figure 3b), and a more negative VG is needed to compensate fixed charges within the NPs and to induce the same density of mobile holes in the channel, leading to a negative ΔVT.22 Under a positive VG (erasing) pulse, trapped holes partly tunnel back into the semiconductor, and Figure 3a shows that the efficiency of this discharging operation is lower than that of the charge-injecting actions. In consequence, the cumulative effect of increasing pulse magnitude leads to a substantial negative shift of VT in all devices, with the slope of ΔVT versus the measurement sequence roughly correlated to the particle size and density. Note that electron trapping is unlikely to be comparable to the hole counterpart due to the negligibly small density of thermal carriers of unintentionally doped pentacene and a large electron injection barrier from Au (ca. 2 eV).23,24 We employed here relatively long pulses (10 s) to maximize the effect of charging/discharging since longer VG pulses induced larger shifts in a transfer characteristic. Note also that the gate control was highly reproducible and there was no noticeable degradation after gate-programming tests, even under prolonged activation with a large VG. The operation mechanism depicted in Figure 3b is close to that of a conventional floating-gate transistor, because NPs function as isolated nanoscale floating gate electrodes. The benefit of our

system as compared to the reported NFGM-type devices is that no tunneling insulator needs to be deposited on Au NPs, because of the insulating properties of αS layers. For a more quantitative description, the extracted maximum threshold voltage shift (ΔVT,max) values were converted into the maximum trapped charge per area (Qmax) by assuming capacitive charging6,8,25 Q max = −C iΔVT,max

(1)

where Ci is the insulator capacitance per area (17.3 nF/cm2). Considering the direction of hysteresis (Figure 2a) and the energetic structure (Figure 3b), major trapped charge carriers are expected to be holes.14,26 The particle density (number of particles per area), N, was estimated from Figure 1d. Now, we can extract the number of holes trapped within a particle, S, by S=

Q max qN

=

−C iΔVT,max qN

(2)

where q is the elementary charge. The results in Figure 3c, d indicate that similar ΔVT,max values found in the 10- and 30-nm high-density NP samples lead to substantially different S parameters. When comparing the samples with tightly packed NPs (5-, 10-, and 30-nm high), we observed a systematic decrease in N and an increase in S with the increasing NP size. This trend corresponds to a higher capacity to accommodate charge carriers in larger NPs. Interestingly, a significantly higher S was found in the 30-nm low-density sample than in the highdensity one. This observation suggests that the particle size is not the sole factor determining the degree of charging, and the interparticle distance plays an additional role; more holes can be injected when the repulsive force exerted by neighboring positively charged NPs is at a longer distance (Figure 3d, inset). We also note that this observation is partly consistent with that reported in the previous report that suggested a discontinuous D

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ACS Applied Materials & Interfaces ID(t ) ∝ t γ

NP network for generating low-leakage, high-efficiency charge trapping elements.14 The volatility of electrical memories is accounted for by various loss mechanisms associated with the diffusion of charges through defects in the blocking material, tunneling, thermionic emission, or combined effects.27 The time scale of charge retention eventually determines a specific target application; thus, we systematically estimated this attribute by an extrapolation-based method.28 To start, a fast transfer sweep was conducted, and the initial current (Iinitial) at VG = 0 V, VD = −20 V was recorded for each sample. Then, the devices were programmed by VG = −50 V, VD = 0 V for 10 s. Note that this operation produces a negative ΔVT, and the magnitude of ID is decreased at VG = 0 V. When VG is released, ID tends to recover its initial value (Iinitial) as charge carriers are slowly detrapped from the Au NPs. We measured this relaxation dynamics by periodically reading ID with VG = 0 V, VD = −20 V, as shown in Figure 4a and Figure S12. As often reported, multiple slopes are identifiable on a log−log plot, which may be related to the physical mechanisms with different time constants.7,8,29 We fit the linear region nearest to the end of the measurement to a power-law relationship

(3)

where t is the relaxation time and γ is the characteristic exponent that corresponds to the slope of the fit lines in Figure 4a and Figure S12. A possible correlation exists between the trends in γ (Figure 4b) and the surface-charge parameters in Figure 3d. It can be inferred that, under a lower density of NPs with a higher population of charges per particle, the charge relaxation can occur more quickly, as there are stronger repulsive interactions within a NP and weaker interparticle confining forces. It is natural to infer that a lower γ would be linked to a longer retention time, however, we found that the extrapolated retention time also reflects the degree of current modulation that is defined as the ratio between Iinitial and the first data point in ID (t) (Figure 4c). It is important to note that by introducing nanosized trapping structures with tailored dimensions and distribution, we realized a device platform that features an exceptionally wide range of data retention time from 103 to 107 s. Among various potential applications, we demonstrate here a bistable programmable/rewritable memory that fully benefits from the short retention and efficient gate control of a 30-nm low-density NP OFET. Figure 4d shows that the device can be read at two well-defined current levels for multiple cycles. In conclusion, we have successfully demonstrated the promising electronic functionality of novel protein-mediated Au NP adsorption systems. The delicate controllability over the protein-NP conjugate monolayer structures has allowed for the successful fabrication of organic transistors with highly tunable memory performance. The mechanisms of charge trapping, retention, and recovery were thoroughly investigated by correlating various structural and electrical parameters. We also emphasize that the thin αS protein layer not only showed active dual functionality (tunneling/blocking) in data programming but also enabled the realization of very high-density largeNP memories without electrical shorting. Our protein−NP hybrid system therefore proved its great potential for various target applications. We fully benefit from the outstanding degree of control over particle dimension and distribution, each of which provides a unique design principle for optimal current levels, hysteresis window, VT control, and retention time. More specifically, small-sized NPs with the low-to-medium distributions will be a proper choice for enabling short-term memory with fast writing/erasing capability, which would be well-suited for neuromorphic applications. In contrast, large-sized NPs with the high-density distribution can be a natural choice for constructing nonvolatile organic flash memories, by virtue of their enhanced charge capacity as well as excellent retention by αS molecular barriers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02268. Experimental details on the preparation and characterization of the αS−Au NP conjugates and OFET devices as well as supplementary DLS, SEM, XPS, and electrical data (PDF)

Figure 4. (a) Transient relaxation measurements after charging the NPs by VG = −50 V for 10 s with VD maintained at zero. The currents (symbols) were recorded at VG = 0 V and VD = −20 V at 20 s intervals. The green dashed line positions the initial current at VG = 0 V and VD = −20 V. Main panel and the inset correspond to the analysis of a 5nm NP sample and a high-density 30-nm NP sample, respectively. (b) Exponent γ of the fits from the analysis shown in a. (c) Gate-pulseinduced current modulation factors and retention time values. (d) Data from the repeated programming/erasing test for a low-density 30-nm NP device. Programming was done by applying VG = −20 V for 10 s (with VD = 0 V) and erasing was done by applying VG = 60 V for 10 s (with VD = 0 V).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. E

DOI: 10.1021/acsami.6b02268 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces *E-mail: [email protected].

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Author Contributions †

C.-H.K., G.B., and J.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2015R1D1A4A01018560), the Center for Advanced SoftElectronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (2011-0031639), Research Program To Solve Social Issues of the NRF funded by the Ministry of Science, ICT and Future Planning (NRF2014M3C8A5030613), two NRF grants funded by the Korean government (MSIP) (2015R1A2A1A15051551, 2012R1A2A2A01013582), and Basic Science Research Program through the NRF funded by the Ministry of Education, Science and Technology (2013R1A6A3A01028589).

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