Biocompatible biologically occurring polymer is suggested as a component of human implantable devices since conventional inorganic materials are apt to trigger inflammation and toxicity problem within human body. Peptides consisting of aromatic amino acid, tyrosine, are chosen, and enhancement on electrical conductivity is studied. Annealing process gives rise to the decrease on resistivity of the peptide films and the growth of the carrier concentration is a plausible reason for such a decrease on resistivity. The annealed peptides are further applied to an active layer of field effect transistor, in which low on/off current ratio (∼10) is obtained.

Proteins are insulator outside but play important roles in electron and proton conduction in biological systems. For example, in photosystem, electrons can be efficiently relayed through the proton coupled mechanism, mediated by redox active tyrosine and proton donating histidine.1 However, translation of such biological principles into synthetic devices is still a challenge probably because of lack of exact understanding and difficulty in control of protein folding. As an effort to make protein based device toward artificial biological system, peptide can be a useful toolkit to simplify the sequence specific behaviors and control the assembly and folding. Based on our previous discovery, the Tyr–Tyr–Ala–Cys–Ala–Tyr–Tyr (YYACAYY) peptide can assemble into flat nano-films which can flatten the top of water droplet into a plane.2 The redox-active tyrosine(Tyr or Y) units in the films were actually able to mediate electron and enhance electrochemical reactions.3 From this observation, we explore the possibility to enhance the electronic conductivity of peptides to advance realization of artificial biological system and also human-implantable devices successfully incorporating semi-conductive or conductive peptides.

Human-implantable devices such as the biosensor and the drug-delivery system have been significantly developed since they have aided medical diagnosing, monitoring, and the delivery of therapeutic materials within the human body.4–6 In detail, implantable devices should function at the targeted organ and be excreted at the end; therefore, biocompatibility and biodegradability become an important issue. The conventional inorganic materials such as silicon and crystalline silicate damage the blood mononuclear phagocytes,7 and inflammation is consequently triggered at the targeted organ.8–11 Even though several methodologies have alleviated the above problem by decreasing the particle size, changing the crystallinity,12 and encapsulating the conventional inorganic materials with synthetic polymers, such efforts complicate the fabrication process, increase the fabrication cost, and pose unintended toxicity problems.13–16 Biologically occurring polymers such as collagen, albumin, chitin, and chitosan can provide a breakthrough since their biocompatibility and biodegradability attributes are intrinsic.17 Therefore, the authors commenced the electrical study of peptides, one of the biologically occurring polymer, as the first step toward implantable devices.

The peptide sequences used in this study contain tyrosine amino acid that can be chemically modified into biocompatible and biodegradable melanin18,19 and can theoretically allow electrons to move freely along the peptides chain through pi-conjugated bonds in phenol. Here, various types of peptides featuring different length and amino acids components were investigated in terms of resistivity with increasing annealing temperature and fabrication process compatibility (Table S1 of the supplementary material). As a result, YYACAYY was finally selected as the starting peptide material to be electrically studied and adopted as an active layer of field effect transistor (FET).

In this paper, electrical conductivity of peptides is improved by inducing in-plane pi-conjugated assembly under reductive gas20,21 through annealing process. The enlarged sp2 hybridized carbon cluster is confirmed by an X-ray Photoelectron Spectroscopy (XPS) result and the electrical improvement is explained by the growth of the carrier concentration through the annealing temperature. Furthermore, annealed peptide was adopted as an active layer of FET, and the on/off current ratio of annealed peptide FETs is demonstrated as functions of the annealing temperature. The device showed possibility to be used as a p-type semiconductor, with an on/off current ratio of 4.6(±1.0) and a field-effect mobility of 4.9(±1.6) × 10−5 cm2 V−1 s−1 for a 600 °C annealed peptide FET. Although poor device performance was obtained, it is meaningful because so far many types of biologically occurring polymers have shown an insulating property,22,23 and annealed materials have shown a conducting property.24,25

The chemical structure of the starting-peptide material, YYACAYY, is depicted in Fig. 1(a). The peptide films were prepared by spin-coating a solution of peptide (1 wt. %) in trifluoroacetic acid (TFA), and the films were annealed at 600 °C, 700 °C, and 800 °C. The schematic diagram of the annealing process is illustrated in Fig. 1(b), and the corresponding Atomic Force Microscopy (AFM) topography of the pristine film and the annealed films at 600 °C, 700 °C, and 800 °C are demonstrated in Figs. 1(c)–1(f), respectively. The thickness is measured by manually scratching the peptide film through cutter, and at least three points are characterized for accuracy. The thickness of the pristine film is ∼66 nm, and the thicknesses of all of the annealed films are under 10 nm. The surface morphology of the pristine film was finely optimized by varying the spin-coating rpm, the peptide concentration (Fig. S1 of the supplementary material), and environmental factors such as the relative humidity (Fig. S2 of the supplementary material) to eliminate the striation effect. As a result, the pristine film achieved a low root-mean-square roughness of 0.434 nm, and the roughness values of all of the annealed films are under a few nm. The detailed values for the thickness and roughness of the annealed films are summarized in Table I.

FIG. 1.

(a) Chemical structure of YYACAYY and (b) illustration of the annealing mechanism in YYACAYY films (model). (c) Surface topology of the pristine film, (d) 600 °C, (e) 700 °C, and (f) 800 °C annealed-peptide films.

FIG. 1.

(a) Chemical structure of YYACAYY and (b) illustration of the annealing mechanism in YYACAYY films (model). (c) Surface topology of the pristine film, (d) 600 °C, (e) 700 °C, and (f) 800 °C annealed-peptide films.

Close modal
TABLE I.

Thickness and roughness of the pristine YYACAYY film and annealed films.

Annealed films
Pristine film600 °C700 °C800 °C
Thickness (nm) 66.1 ± 3.0 4.6 ± 0.9 2.3 ± 0.3 2.0 ± 0.3 
Roughness (nm) 0.434 0.682 2.91 3.21 
Annealed films
Pristine film600 °C700 °C800 °C
Thickness (nm) 66.1 ± 3.0 4.6 ± 0.9 2.3 ± 0.3 2.0 ± 0.3 
Roughness (nm) 0.434 0.682 2.91 3.21 

Raman spectrum is shown in Fig. 2(a), in which the pristine film does not show any obvious peak, whereas the annealed films all show a clear D peak (1355 cm−1) and a G peak (1574 cm−1), which implies a carbonaceous-material state. The D peak corresponds to the A1g breathing mode of the six-atom ring that is considered as a structural imperfection that is caused by a defect. The G peak is attributed to the E2g stretching mode that is caused by the in-plane sp2-carbon vibration of a perfect graphite crystal.26,27 The intensity ratio between the D and G peaks (ID/IG) declines gradually as the annealing temperature rises (Table II), and this means that graphite-like sp2 hybridized-carbon clusters become larger.28,29

FIG. 2.

(a) Raman-spectroscopy results of pristine and annealed YYACAYY films. (b) XPS results of C 1s, (c) N 1s and (d) S 2p for the pristine film and annealed film.

FIG. 2.

(a) Raman-spectroscopy results of pristine and annealed YYACAYY films. (b) XPS results of C 1s, (c) N 1s and (d) S 2p for the pristine film and annealed film.

Close modal
TABLE II.

Raman-spectroscopy data and optical bandgap of the annealed films.

Annealed films
600 °C700 °C800 °C
ID/IG 1.66 1.60 1.43 
Optical bandgap (eV) 2.55 2.47 2.21 
Annealed films
600 °C700 °C800 °C
ID/IG 1.66 1.60 1.43 
Optical bandgap (eV) 2.55 2.47 2.21 

Figures 2(b)–2(d) represents XPS spectra of the carbon, nitrogen, and sulfur, respectively. The C 1s spectra have generally been interpreted that the C=C peak at 284.5 eV corresponds to the sp2 hybridized carbon, and the other peaks located in the higher binding-energy levels (C–N, C–S, or C–O at ∼286.2 eV, C=O at ∼287.8 eV) are related to the heteroatom-conjugated carbon.28–30 In Fig. 2(b), the heteroatom-conjugated carbon peaks at the higher binding energy almost disappear after the annealing process, while the intensity of the sp2 hybridized-carbon peak steadily rises along with the increasing of the annealing temperature; therefore, it is certain that the sp2 hybridized-carbon cluster is enlarged. Such a result is a strong evidence of increase on G peak in Raman spectra and plausibly explains the small value of ID/IG obtained at the 800 °C annealed film.

The XPS spectra of the nitrogen are presented in Fig. 2(c), in which the pristine film originally has one sharp peak (∼399.7 eV) that is the identification of the free-amino groups,31 while two totally different peaks corresponding to pyridinic N (∼398.5 eV) and pyrrolic N (∼400.2 eV) appear at the annealed films;30 this means that the nitrogen-engaged functional groups were definitely changed from amino-acid nitrogen to pi-conjugated nitrogen. The intensity of the pi-conjugated nitrogen steadily decreases as the increasing annealing temperature, and it may have impact on decrease on D peak in Raman spectra. The XPS spectra of the sulfur are presented in Fig. 2(d), showing only one sharp peak (C–S at ∼163.9 eV) in the pristine film, however, thiophene-sulfur doublet (C–S at ∼163.9 eV, C=S and ∼165.1 eV) and oxidized sulfur group such as sulfate or sulfonate (C–SOx–C, x = 2-4, at 167.5-171.5 eV)28,32 were observed after the annealing process. Change on chemical functional groups is the evidence of formation of pi-conjugated sulfur after the annealing process and oxidized sulfur might improve electrical and electro-chemical property.28 

The resistivity of the pristine film and annealed peptide films are characterized under the vacuum condition in Fig. 3(a). Au electrodes were deposited on all the films using shadow mask, and the resistivity of the films were measured. The pristine film has resistivity of ∼108 (Ω cm), which is in the range of typical insulator, however, resistivity decreased as increasing annealing temperature. The resistivity of the peptide films annealed at 600 °C, 700 °C, and 800 °C is achieved as ∼102 (Ω cm), 100 (Ω cm), and 10−2 (Ω cm), respectively. Resistivity of peptides annealed at 300 °C, 400 °C, and 500 °C is also calculated, however, they are excluded for further investigation due to high resistivity ∼108 (Ω cm) (Fig. S3 of the supplementary material). Based on Hall effect measurement data shown in Fig. 3(b), carrier concentration rises, while mobility decreases with respect to annealing temperature. Considering resistivity is reversely proportional to carrier concentration and mobility, decrease on resistivity is attributed to the growth of the carrier concentration by ∼105, which is far larger than the decrease on mobility by ∼102. The carrier concentration is theoretically related to the bandgap of a material; therefore, the optical bandgaps of the annealed films are extracted from the Tauc plot elaborated in Fig. 3(c). Ultraviolet- Visible-Near Infrared (UV-Vis-NIR) spectrophotometer was used for the obtaining of the transmittance data of the annealed films on quartz (Fig. S4 of the supplementary material), and the Tauc plot was drawn to obtain the optical bandgap of the films based on the following equation:

αω=const(ωEopt)n,
(1)

where α, ω, Eopt, and n are absorption coefficient, photon energy of incident light, optical bandgap, and the number characterizing the optical-absorption processes, respectively.33–35 The value of 1/2 was selected for n due to the directly allowed transition that is caused by the amorphous structure of the pristine film and the annealed films (Fig. S5 of the supplementary material). The optical bandgaps of the films annealed at 600 °C, 700 °C, and 800 °C are 2.55 eV, 2.47 eV, and 2.21 eV, (Table II) respectively, and based on Eq. (1), such a decrease on bandgap certainly causes the increase of the carrier concentration.

FIG. 3.

(a) Resistivity of pristine and annealed films, (b) carrier concentration and the mobility of the annealed peptide films as a function of the annealing temperature. (c) Tauc plot of annealed YYACAYY films with respect to annealing temperatures, in which optical bandgap of each annealed film was extracted by linear extrapolation of the Tauc plot. (d) Resistivity of the annealed films with respect to measurement temperature.

FIG. 3.

(a) Resistivity of pristine and annealed films, (b) carrier concentration and the mobility of the annealed peptide films as a function of the annealing temperature. (c) Tauc plot of annealed YYACAYY films with respect to annealing temperatures, in which optical bandgap of each annealed film was extracted by linear extrapolation of the Tauc plot. (d) Resistivity of the annealed films with respect to measurement temperature.

Close modal

Each annealed-peptide film with different measurement temperatures is also electrically characterized in Fig. 3(d) and the resistivity of each film decreased as the measurement temperature was increased; this tendency is the same as that of the conventional-semiconductor characteristics and is called a “negative Temperature Coefficient of Resistivity (TCR).”36 Based on this result, the FET for which the annealed-peptide films are used as an active material was fabricated.

The electrodes of the transistors are composed of tapered molybdenum, and the annealed peptides serve as an active-channel material. Thermally grown SiO2 was used for the dielectric material, and highly p-doped silicon was utilized for the global gate. The inset of Fig. 4(d) presents the schematic diagram of the fabricated device. Figures 4(a)–4(c) illustrate the transfer curves of the 600 °C, 700 °C, and 800 °C annealed peptide FETs, respectively, when the drain voltage is biased at −10 V under the vacuum condition. The gate voltage bias was swept from a positive to a negative voltage, and all of the annealed-peptide FETs seem to show the p-type-semiconductor behavior that features a higher drain current at a negative gate voltage. All of the devices have a W/L of 30 μm/100 μm and gate leakage current of all the devices was under 10−8 A. The on/off current was calculated as a function of the annealing temperature, as shown in Fig. 4(d), in which the ratio decreased from 4.6(±1.0) to 1.2(±0.02) as the annealing temperature was increased from 600 °C to 800 °C. Such a small gating effect was repeatedly confirmed by measuring over three devices and this tendency is also confirmed by the output curves of the devices (Fig. S6 of the supplementary material). FETs incorporating 800 °C annealed peptide have resistor-like output curve, implying that the film function like a conductor not as a semiconductor. Contrarily, 600 °C annealed peptide shows small gating effect, and it records field-effect mobility of 4.9(±1.6) × 10−5 cm2 V−1 s−1. Whether the annealed films operate as a semiconductor might be attributed to carrier concentration of the films previously elaborated in Fig. 3(b), in which 700 °C and 800 °C annealed peptides contain carriers more than 1020 cm−3 that is far bigger than that of general silicon37 and oxide semiconductor.38,39 In addition, p-type characteristics at 600 °C annealed peptide might be explained by the hetero-atom conjugated carbon cluster explained in Fig. 2. Objectively, the on/off current ratio is too poor in the current state, whereas it is meaningful in that annealed peptide can be a candidate material for an active layer of FET by further engineering carrier concentration and hetero-atom doping characteristics.

FIG. 4.

(a) Representative transfer curves of YYACAYY FET annealed at 600 °C, (b) 700 °C, and (c) 800 °C when Vd is biased at −10 V. The devices all have a 100 μm width and a 30 μm length. (d) On/off current ratio extracted from above-mentioned graphs is shown as a function of the annealing temperature. The schematic diagram for the devices is shown in the inset.

FIG. 4.

(a) Representative transfer curves of YYACAYY FET annealed at 600 °C, (b) 700 °C, and (c) 800 °C when Vd is biased at −10 V. The devices all have a 100 μm width and a 30 μm length. (d) On/off current ratio extracted from above-mentioned graphs is shown as a function of the annealing temperature. The schematic diagram for the devices is shown in the inset.

Close modal

In summary, peptide has its intrinsic biocompatibility and biodegradability, being selected as a starting material for the realization of human implantable devices. Insulating property of the peptide film limits the scalability into an implantable device; therefore, the improvement on electrical conductivity of the peptide film is studied. In-plane pi-conjugated assembly is induced through the annealing process and expanded sp2 hybridized carbon cluster is identified by the XPS result. The resistivity of annealed films steadily decreases with respect to annealing temperature, and the tendency is explained by the rise of carrier concentration. The annealed peptides are further adopted as an active-channel material of FET, and the devices seem to function as a p-type semiconductor. The on/off current ratio of 4.6(±1.0) was achieved at the 600 °C annealed-peptide FET, and the decreasing tendency of the on/off current ratio according to the annealing temperature is elaborated by the amount of the carrier concentration. The on/off current ratio is poor in the current state; however, the possibility for annealed peptide to be applied to an active material of FET is demonstrated. Future work will focus on controlling carrier concentration toward attaining high on/off current ratio through the regulation of annealing temperature and peptide sequence identification.

See supplementary material for the experimental procedures, resistivity according to peptide sequence, peptide surface morphology optimization, transmission data, XRD data, and additional device property of the annealed peptide film.

This work was supported by Samsung Research Funding Center of Samsung Electronics under Project No. SRFC-MA1401-01.

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Supplementary Material