· Web viewSemiconducting 1D CNT channels are used with a 2D hexagonal boron nitride (h-BN) gate dielectric and traces of silver nanowires as the conductive electrodes; all deposited

Flexible, Print-in-Place 1D-2D Thin-Film

Transistors Using Aerosol Jet Printing

Shiheng Lu1, Jorge A. Cardenas1, Robyn Worsley2, Nicholas X. Williams1, Joseph B. Andrews1,

Cinzia Casiraghi2 and Aaron D. Franklin*, 1, 3

1Department of Electrical and Computer Engineering, Duke University, Durham NC 27708,

USA

2School of Chemistry, University of Manchester, Manchester M13 9PL, United Kingdom

3Department of Chemistry, Duke University, Durham NC 27708, USA

TABLE OF CONTENTS GRAPHIC

ABSTRACT

Semiconducting carbon nanotubes (CNTs) printed into thin films offer high electrical

performance, significant mechanical stability, and compatibility with low-temperature

processing. Yet, the implementation of low-temperature printed devices, such as CNT thin-film

transistors (CNT-TFTs), has been hindered by relatively high process temperature requirements

imposed by other device layers—dielectrics and contacts. In this work, we overcome temperature

constraints and demonstrate 1D-2D thin-film transistors (1D-2D TFTs) in a low-temperature

(maximum exposure ≤ 80 °C) full print-in-place process (i.e., no substrate removal from printer

throughout the entire process) using an aerosol jet printer. Semiconducting 1D CNT channels are

used with a 2D hexagonal boron nitride (h-BN) gate dielectric and traces of silver nanowires as

the conductive electrodes; all deposited using the same printer. The aerosol jet-printed 2D h-BN

films were realized via proper ink formulation, such as utilizing the binder hydroxypropyl

methylcellulose, which suppresses re-dispersion between adjacent printed layers. In addition to

an ON/OFF current ratio up to 3.5 ×105, channel mobility up to 10.7 c m2 ·V −1 · s−1, and low gate

hysteresis, 1D-2D TFTs exhibit extraordinary mechanical stability under bending due to the

nanoscale network structure of each layer, with minimal changes in performance after 1000

bending test cycles at 2.1 % strain. It is also confirmed that none of the device layers requires

any high-temperature treatments to realize optimal performance. These findings provide an

attractive approach towards a cost-effective, direct-write realization of electronics.

KEYWORDS

carbon nanotubes, hexagonal boron nitride, thin-film transistors, silver nanowires, low-

temperature printing, aerosol jet printing, flexible electronics

The use of printing technologies for flexible electronics has rapidly increased in

prominence due to its simplicity, low-cost, and compatibility with a broad range of materials.1

These advantages have made printed electronics an attractive solution for the development of

both large-area and direct-write applications.2-4 Printed electronics consists of the transfer of a

functional material-based ink onto a substrate via either template-based methods such as spray-

coating5, 6 and roll-to-roll printing,7, 8 or direct-writing methods including inkjet6, 9 and aerosol jet

printing.10 Currently, there exists a wide variety of printable inks made of organic-based

materials,11 metal oxides,12 metallic nanostructures13 and carbon-based materials.14 Organic

electronic materials are typically semiconducting and suffer from low carrier mobility and poor

air stability.15 Metal oxide semiconducting inks usually require high process temperatures, which

limits the choices of printing methods and substrates. In contrast, semiconducting carbon

nanotubes (CNTs) exhibit excellent electronic, chemical, and mechanical properties, including

high carrier mobility, air stability, flexibility, and large surface-to-volume ratio, with minimal

thermal treatment required.16-18 Combined, the stability as well as the minimal temperature

requirements allow more freedom in substrate and material selection as well as compatibility

with simplified and low-cost printing techniques.19 These beneficial features of CNTs enable a

wide range of functional applications, from display backplanes20 and logic circuits14, 21 to

environmental22, 23 and biological sensors.24

Significant research effort has been focused on improving printed carbon nanotube thin-

film transistors (CNT-TFTs), culminating in better performance,25, 26 a wide range of

functionalities,21, 22, 24 and reduced fabrication costs.13 Although there have been a multitude of

demonstrations involving the direct-write printing of various nanomaterial inks and devices, the

device fabrication still requires processing outside the printer, making the overall manufacturing

process more time consuming and expensive. A completely in-place printed transistor process

has not yet been realized, largely owing to the significant challenges of ink layer compatibility,

reliance on sintering, and lack of sufficient quality low-temperature dielectrics. Recently, an

aerosol jet-based in-place printing technique for substrate-gated CNT-TFTs demonstrated

promising potential for cost-effective fabrication of semiconducting and conducting layers.13

While free from any postprocessing outside the printer, the CNT-TFTs were made on a Si

substrate with thermally grown SiO2 dielectric as the gate stack, limiting the number of device

structures and applications owing to the rigid and global substrate gate. To obtain an equally

simple printing method with fewer limitations, an in-place printable gate stack is required.

While silver nanowire (AgNW) inks have been proven viable for low-temperature printed

electrodes,27-29 an equally suitable gate dielectric that can also be printed and cured at low

temperatures has remained elusive. Common dielectric materials for printed CNT-TFTs include

ion gels,30 insulating polymers,25 and polymer/nanofiller hybrid dielectrics, among which

BaTiO3/poly(methyl methacrylate) has been widely used.31-33 To realize optimal performance,

polymers and hybrid dielectrics require high-temperature post-processing, above what a typical

printer can provide,25 thus rendering them incompatible with a fully in-place printed TFT. Ion

gels can be printed at low temperature, yet they suffer from limited switching speed that is

dependent on ionic motion and often yield ambipolar transistor behavior.8, 34

One promising candidate for a printable insulating dielectric can be found in recently

developed 2D material-based inks. The demonstration of large-scale liquid phase exfoliation

(LPE) of monolayer graphene initiated the use of these printable materials.35 Since then, multiple

2D materials, including graphene,36, 37 transition metal dichalcogenides (TMDs),38, 39 hexagonal

boron nitride (h-BN),6, 37, 40-42 and black phosphorus (BP)43 have been developed into inks for

electronic applications, such as semiconducting channels,5 conducting electrodes,6, 42, 44-46 and

insulating dielectrics.6, 40, 42 Notably, in 2017, McManus et al. proposed a protocol to prepare

water-based 2D material inks for direct-write printing.38 Such inks provide an attractive solution

for the problems frequently encountered during LPE methods, such as solvent toxicity and high

process temperature. Printed 2D material films exhibit extraordinary properties of flexibility and

environmental stability;36, 40 yet, their electronic properties are typically worse than those of the

1D alternatives. For example, the conductivity of printed graphene and mobility of

semiconducting MoS2, are significantly lower than their 1D alternatives such as AgNWs and

semiconducting CNTs, respectively.5, 8, 27, 36 Even still, there are other electronic properties, such

as the dielectric performance of h-BN thin films,42 that are of high enough importance to

motivate further investigation. Designed initially for inkjet printing, water-based h-BN ink

presents thermal and dielectric properties that show promise for print-in-place applications using

aerosol jet-printing—41, 42 a technology with several advantages compared to standard inkjet

printing.

Aerosol jet printing includes a sheath gas flow that impedes the direct contact between

the ink and the sidewall of the nozzle, thus preventing the ink from blocking the nozzle (Fig. S1),

which is one of the major issues encountered by inkjet printing. In addition, an aerosol jet printer

is much less demanding on ink rheology than a commonly used piezoelectric inkjet printer,38

allowing for wide ink compatibility and rigorous ink formulation tuning. Even though these

features make aerosol jet printing especially suitable for 2D materials, aerosol jet printing of 2D

material-based inks has been barely demonstrated.47

In this study, we realize the aerosol jet printing of water-based h-BN inks and

demonstrate a fully printed, top-gated and flexible 1D-2D thin-film transistor (1D-2D TFT). The

1D-2D TFT exploits 1D and 2D nanostructures with the best properties for a transistor, including

h-BN used as a gate dielectric, a semiconducting CNT channel, and AgNWs as source, drain,

and gate electrodes. Each component is deposited through aerosol jet printing (Fig. 1), and the

highest temperature used during the entire printing process is 80 °C on the printer platen, which

is the lowest temperature reported for printing CNT-TFTs to date (Table S1). The dielectric

properties of the resultant films were investigated via profilometry and capacitance

measurements, and a clean h-BN/AgNW interface was confirmed using scanning electron

microscopy (SEM). While benefiting from the convenience of low-temperature steps, the

resulting performance of the 1D-2D TFTs is on par with previously reported fully printed TFTs,

with high ON-OFF current ratio, mechanical flexibility, and minimal hysteresis. The short-term

and long-term effects of a 150 °C annealing are examined, revealing that such a postprinting

treatment is nonessential for maximizing the device performance. Furthermore, we demonstrate

the ability to realize these flexible 1D-2D TFTs with a full print-in-place process, without any

external baking, washing, or coating steps away from the printer.

RESULTS AND DISCUSSION

The fabrication process of a flexible 1D-2D TFT on a polyimide (Kapton) substrate is

illustrated in Fig. 1a, with detailed description in the Materials and Methods section. In brief, the

cleaned and functionalized substrate was loaded onto the platen, and CNT ink (99.9%

semiconducting, 0.05 mg/mL concentration) diluted from commercial ink (IsoSol-S100,

NanoIntegris) was printed onto the channel region at room temperature. The channel region was

rinsed with toluene either externally or in-place, resulting in two different process flows. The

external rinsing process was carried out in a fume hood where the channel region was first rinsed

with room-temperature toluene followed by a 70 °C toluene bath to aid in the removal of

polymer surfactant. In processing some of the devices, the rinsing and toluene bath step was

replaced by a simplified in-place alternative during which the platen temperature of the printer

was increased to 80 °C, and a brief toluene rinse was applied onto the channel region for 20 s

(see Video S1). The resulting CNT network is illustrated in Fig. 1d and has a CNT film density

around 13 nanotubes/µm2. Note that once the sample was placed onto the printer, only the

external rinsing step was carried out away from the printer, which means that for the process

flow with in-place rinsing, the whole printing process was carried out in-place without removal

of the substrate from the platen.

Figure 1. Flexible and fully print-in-place 1D-2D TFTs (steps II – VI are performed without removing substrate from printer platen). a) Schematic illustration of the fabrication process flow including: I) substrate preparation with poly-L-lysine (PLL) functionalization, II) printing the carbon nanotube (CNT) channel, III) rinsing the CNT thin-film channel, IV) printing the AgNW

source/drain (S/D) electrodes, V) printing the h-BN gate dielectric layer, and VI) printing the AgNW gate electrode. b) Photo and c) optical image of TFTs after the fabrication process. d) Atomic force microscopy (AFM) image of a printed CNT channel after rinsing.

After the CNT channel was printed and rinsed, AgNW source/drain (S/D) electrodes, h-

BN dielectric, and AgNW gate electrodes were then sequentially printed with the corresponding

as-prepared inks. The platen temperature was kept at 80 °C during the S/D and gate stack

processes. The AgNW and h-BN nanostructures were prepared as described in references27, 42

with a concentration of 10 mg/mL and 2 mg/mL, respectively. Because of the low temperatures

available for in-place printing and the aqueous nature of h-BN and AgNW inks, remixing at the

h-BN/AgNW interface, caused by the nonorthogonal solvent systems,48 is a significant concern,

likely to result in inferior interfacial definition and ultimately device failure. Typically, binders,

such as cellulose36, 40 or gum,38, 42 are added to each ink to hinder the interfacial redispersion of the

constituent materials. In this work, propylene glycol and hydroxypropyl methylcellulose

(HPMC) were added as a secondary solvent and binder, respectively, to modify the inks’

rheological properties (Fig. S3). We used HPMC as the binder due to its water solubility,

biocompatibility, and ability to introduce rheology modification at low concentration. The

improved rheological properties, especially a higher viscosity, served to impede the diffusion of

materials, thus alleviating the potential for interfacial redispersion. 38, 40, 42 Furthermore, in

addition to increasing the ink viscosity, the presence of propylene glycol in aqueous inks also

manipulates the Marangoni flow, which further suppresses an unwanted coffee-ring effect (Fig.

S3c).38, 49 Detailed ink preparation and formulation is described in the Materials and Methods

section.

As this low-temperature printed AgNW/h-BN interface is of great significance to the

operation of the ultimate 1D-2D TFTs, it was studied in detail. To do so, capacitor-like

heterojunctions of AgNWs and h-BN were printed on SiO2 (300 nm)/Si substrates. Film

morphology as well as the cross-sections of various regions of the junctions were studied using

an SEM (Fig. 2). The capacitor cross-section (Fig. 2i) indicates clear interfaces between the h-

BN and both the top and bottom AgNW films. No substantial remixing at the interfaces was

observed despite the fact that both the AgNW and h-BN inks are water-based—a major

milestone in realizing compatible, in-place printing of such diverse layers (Fig. S4). It is also

noticeable that the AgNW film directly printed on the SiO2/Si substrate (Fig. 2f, g) shows a film

uniformity higher than that of the AgNW layer on top of the h-BN film (Fig. 2i, j). A possible

cause is the surface roughness of the h-BN film (Fig. 2d, e), which is a result of overspray and

aggregation of the 2D nanoflakes. A study of the film thickness was carried out via profilometry

measurements (Fig. S6), and the results are consistent with SEM imaging (Fig. 2i). Additionally,

a linear relationship between the number of printing passes and the film thickness was confirmed

(Fig. S6c).

Figure 2. Fully printed h-BN capacitor structures. a) and b) Photographs of printed AgNW/h-BN/AgNW heterostructures with colored dots indicating locations for SEM images. c) Schematic diagram illustrating the cross-section of the printed stack and locations of SEM images. d-e) Top-view SEM images of h-BN film printed directly on SiO2/Si substrate with individual h-BN flakes visible. SEM images of AgNW film printed directly on SiO2/Si substrate from both f) cross-sectional and g-h) top views. SEM images of the AgNW/h-BN/AgNW stack from both i) cross-sectional and j-k) top views.

The dielectric properties of the h-BN network were studied using a capacitor structure

fabricated on a Kapton substrate (Fig. 3) to avoid parasitic capacitance from silicon. Large Cr (5

nm) /Au (70 nm) pads were evaporated onto 127-µm-thick Kapton substrates and utilized as

bottom electrodes to minimize the impact of interface roughness (Fig. S5). After the h-BN was

printed, AgNW networks were printed as the top electrodes with an effective area of 0.664 mm2.

Capacitors of increasing h-BN layers/thicknesses were printed and then analyzed, showing an

inverse linear dependence between the film thickness and capacitance (Fig. 3c), as expected. The

ideal capacitance behavior over the frequency range between 1 kHz and 1 MHz was confirmed

via impedance spectroscopic analysis (Fig. S7). A dielectric constant ϵ r ≈ 3.01 was extracted

using the standard parallel plate capacitance equation: C=ϵ r ϵ 0 A / t . This relative permittivity is

lower than that of bulk h-BN material, which has been found to have ϵ r ≈ 4.2.50 Some previous

reports on printed h-BN films present results similar to this work,6 whereas others report slightly

higher values.40, 42 The difference in dielectric constant could either originate from the film

porosity, ink additives, flake orientation, 42 electrode coverage, or the extra capacitance from the

porosity of AgNW films (Fig. 2k).

Figure 3. Dielectric properties of aerosol jet printed h-BN thin film. a) Schematic diagram showing the capacitor structure. b) Small signal capacitance per unit area versus frequency of gold/h-BN/AgNW capacitors with varied dielectric thicknesses determined by the number of print passes. DC bias was set at 2 V and a small AC signal amplitude of 50 mV was applied. c) Linear fitting between the reciprocal of printed h-BN dielectric thickness and the corresponding film capacitance. Each capacitance is the average from three capacitors, with error bars denoting standard deviation, and the thickness is determined via profilometry measurements shown in Fig S6.

With the properties of the printed 2D h-BN dielectric film understood, the layer was

employed as the gate dielectric for a fully printed CNT-TFT; with a channel of 1D CNTs,

dielectric of 2D h-BN, and contacts of colloquially 1D AgNWs, the resultant device is referred to

as a 1D-2D TFT (Fig. 1). The electrical performance of a representative printed 1D-2D TFT,

including the subthreshold, transfer, and output behavior, is demonstrated in Fig. 4a and 4b. The

ON-OFF current ratio of the examined device is I ON / IOFF ≈ 6 ×104 with an ON-state current

I ON=0.78 μA. The channel mobility is approximately μ=4.18c m2 ·V −1 · s−1 and is extracted

using the equation μ=(L¿¿CH g¿¿ m)/(W CH∗COX∗V DS)¿¿, where LCH and W CH are the length

and width of the channel, respectively, gm is the peak transconductance, V DS is the voltage

between the drain and source electrodes, and COX=1.04 nF·c m−2 is the dielectric capacitance per

unit area extracted from the fitting curve in Fig. 3c with 3 printing passes. The average +/-

standard deviation for mobility and ON/OFF current ratio are μ=4.0 ± 2.6 cm2 ·V −1 · s−1 and

log (ION / IOFF)≈ 4.76 ± 0.42, respectively, with the best performance achieved on different

devices reaching μm=10.2c m2 · V−1 · s−1 and (I ON / IOFF)m ≈ 3.5 ×105. Gate leakage is on the same

order of magnitude as the noise level (Fig. S8).

It has been shown that the size of constituent nanomaterials within a thin film will often

be a significant factor in determining the electrical properties of the film.51, 52 This is based on the

dominant contribution of carrier transport at the junctions between nanomaterials. In the case of

the semiconducting CNT thin-film channels in this work, the diameter range and the average

length of the nanotubes were 1.2 – 1.4 nm and 2 µm, respectively, thus resulting in a film that is

undoubtedly dominated by percolation transport across junctions. Note that Fig. 1d suggests a

relatively low CNT density (~13 CNTs/µm2) compared with many previous works on CNT-

based networks,8, 53, 54 , thus the parallel capacitance extracted from Fig. 3c would be over-

estimated. Therefore we adopted a more rigorous model on gate-CNT coupling,21, 55 which gives

a slightly higher mobility estimation of μ=4.2 ±2.7 cm2 · V−1 · s−1 and μm=10.7 cm2 ·V −1 · s−1 (See

Note S1).

The only non-printing step required for fully print-in-place 1D-2D TFTs is the rinsing of

the CNT layer after it is printed. This step can either be completed external from the printer

followed by a bath in hot toluene, or in situ on the printer without moving the substrate to enable

a full print-in-place process. Recent work suggested that in-place rinsing resulted in a CNT film

with electrical properties that are on par with those of externally rinsed films.13 A similar

comparison was made for these 1D-2D TFTs. Subthreshold and transfer characteristics of an in-

place rinsed (fully print-in-place) device and an externally rinsed (normally printed) device are

shown in Fig. 4c, with more analysis of print-in-place devices provided in Fig. S9 and S10b. It is

noticeable that 1D-2D TFTs with either rinsing step give a similar ON-current and I ON / IOFF ,

whereas the threshold voltage of the in-place rinsed device is slightly more negative. These

results show that in-place printing can be used to fabricate 1D-2D TFTs in a simpler fashion

without sacrificing device performance.

Figure 4. Electrical performance of fully printed 1D-2D TFTs. a) Transfer (solid lines) and subthreshold (dashed lines) characteristics of a printed 1D-2D TFT with external rinsing. b) Output characteristics of the same device in a). c) Comparison of transfer (solid lines) and subthreshold (dashed lines) characteristics between in-place rinsed and externally rinsed 1D-2D TFTs.

Although gate hysteresis has been a major obstacle to more prevalent applications of

CNT-TFTs,25 the printed 1D-2D TFTs exhibit little hysteresis, as is shown in Fig. S10. Among

11 TFTs analyzed, the maximum and minimum values of hysteresis extracted are 4.70 and 0.16

V respectively, with an average value of 1.5 ± 1.6 V. Most devices show hysteresis below 1V

over an 80 V gate swing range (i.e., hysteresis of < 1.3% of the gate swing). No substantial

difference between in-place rinsed TFTs and externally rinsed TFTs is observed for hysteresis.

Such performance is attributed to the top-gate device structure as the dielectric/ambient

interfacial traps are a dominant cause of gate hysteresis,56 and the gate stack serves as an

encapsulant against ambient conditions.57 Hysteresis becomes smaller under higher scan rate (as

seen in Fig. S10c), which is due to the rate limiting behavior of charge traps. 57

To ensure that a high-temperature treatment is not necessary for the 1D-2D TFTs, the

effect of annealing on the printed devices was also studied. In Fig. S11, the subthreshold

(transfer) characteristics of a 1D-2D TFT before, right after, 1 day after and 3 days after a 150 °C

annealing in a vacuum oven (Isotemp 281A) are demonstrated. Surprisingly, both the threshold

voltage and transconductance exhibited a small negative shift right after annealing, and then both

gradually stabilized at their initial states. Despite a small increase in the transconductance after

the annealing-and-recovery process, possibly due to the decompositions of impurities, it is safe

to state that annealing does not have any significant long-term effects on the printed 1D-2D

TFTs. That is very different from many commonly used printable dielectric materials, such as

polymers, which usually go through a high-temperature treatment during the fabrication process

in order to yield their best possible characteristics. The temporary negative shift in threshold

voltage and transconductance right after annealing could either originate from the channel or the

gate stack, and although it is not going to be discussed in detail in this study, it is an interesting

topic for future research. Ultimately, this finding that a high temperature annealing of a printed

h-BN film does not provide any performance enhancement is quite transformative for the printed

electronics space and particularly for direct-write, print-in-place approaches.

The mechanical flexibility of printed 1D-2D TFTs is shown in Fig. 5. The devices exhibit

insignificant change in subthreshold (transfer) performance after 1000 bending cycles at 3-mm

radius and survive a bending radius as low as 2 mm (3.2% strain), demonstrating impressive

stability under mechanical flexing. Such flexibility is enabled by the network structure of the

nanotubes, h-BN nanoflakes and the nanowires—a direct benefit of the 1D-2D nature of these

TFTs. Interestingly, the ON-current of TFTs even increased slightly when the bending radius

was ~3 mm; however, the performance did begin to decrease at smaller bending radii (Fig. 5a).

Such phenomenon may be attributed to a widened channel and a stretched dielectric layer under

moderate tensile stress (below 2.1%) along the S/D electrodes. Due to the negative correlation

between strain ε and substrate thickness t, implementing thinner substrates is a practical way to

achieve smaller bending radii. For the same TFT on a 25 µm-thick substrate, for instance, a

conservative estimation of the bending radius limit would be around 0.4 mm (Fig. 5c) according

to a simplified model of ϵ=t /2 r. A comparison of the bending performance of these 1D-2D

TFTs against other printed, CNT-based TFTs is provided in Table S1; note, there has yet to be a

particular metric that is consistently reported for benchmarking performance under strain, so the

maximum demonstrated strain is simply provided for comparison. Future work should be done

with these and other printed, flexible devices to ascertain the strain limit that the devices can

endure. 1D-2D TFTs could also be printed on flexible substrates other than Kapton, as we

demonstrate with devices printed on paper, which is low-cost and relatively rough (not electronic

grade or designed for printed electronics) (Fig. S12). As shown in Fig. S12b, despite a higher

OFF-state leakage current, which is attributed to the substrate roughness, a decent gate

modulation is achieved, with an ON-OFF current ratio higher than 1000. The compatibility of

1D-2D TFTs with low-cost paper substrates exemplifies the significant applicability of this low-

temperature, in-place printing process.

Figure 5. Flexibility test of printed 1D-2D TFTs. Subthreshold characteristics of a printed 1D-2D TFT taken while a) the device is flexed at different bending radii and b) after different numbers of bending cycles with a bending radius of 3 mm (~2.1% strain). c) Estimated strain with respect to bending radius for various substrate thickness. The green dashed line gives the strain when the 127µm substrate from this work is bent at 2 mm radius. The inset is a photograph of 1D-2D TFTs bent at a radius of 3 mm.

To benchmark the performance of in-place printed 1D-2D TFTs enabled by the

utilization of aerosol jet-printed h-BN, a comprehensive comparison between the 1D-2D TFTs

and other printed flexible CNT-TFTs in the literature8, 14, 25, 30-33 is shown in Fig. 6. In general,

although printed-in-place at the lowest overall process temperature of any CNT-TFT printed to

date, the devices reported in this work yield high performance relative to previous reports. As

presented in Fig 6a, the 1D-2D TFTs exhibit mobility comparable with most CNT-TFTs utilizing

common dielectrics (e.g., BaTiO3 or polymers). The high ON/OFF current ratio (Fig. 6b) is

partly attributed to the relatively low CNT film density. While a significantly higher mobility is

achievable, this is usually realized by drastically increasing the density of the CNT networks,

which would inevitably compromise the ON/OFF current ratio (as seen in ref. 30).

Figure 6. Benchmarking performance of 1D-2D TFTs with previously reported printed, flexible CNT-TFTs. Comparison between the maximum process temperature, (a) effective hole mobility, and (b) ON/OFF current ratio. Diamonds refer to the average or the typical results, whereas circles refer to the best results reported.

CONCLUSION

In summary, we demonstrated a fully in-place aerosol jet-printed 1D-2D TFT structure

using a CNT channel, a h-BN dielectric layer, and AgNW electrodes at the lowest overall

process temperature ever reported (maximum process temperature of 80 °C). The use of 1D and

2D nanomaterials with the best properties for transistor operation allowed us to achieve

outstanding performance, even under bending conditions. In addition to in-place printing, a low

processing temperature is also favorable in that it makes a step toward direct-write electronics

onto thermally sensitive surfaces. The printing process could be carried out on low-cost flexible

substrates, including paper. It was discovered that the printed h-BN dielectric does not benefit

from subsequent annealing or other high-temperature treatments for optimizing dielectric

performance—a major finding for realizing low-temperature printed dielectric films. These

discoveries boost the development of flexible electronic device fabrication techniques toward a

simple, low-cost, and fully print-in-place approach.

MATERIALS AND METHODS

Substrate Cleaning and Functionalization

Substrate cleaning of SiO2 (300 nm)/Si and Kapton (127 µm) includes a 5-min

ultrasonication in acetone and isopropyl alcohol (IPA) each, followed by a deionized (DI) water

rinse and a nitrogen blow drying. Substrates were then immersed in 0.1% w/v poly-L-lysine

(PLL) / water solution (Sigma-Aldrich) for 5 min, rinsed with DI water, and dried in nitrogen to

increase CNT adhesion. Substrate cleaning of paper (Mohawk Color Copy Gloss, Pure White, 80

text) includes a 5 min ultrasonication in acetone followed by an IPA rinse and nitrogen blow

drying.

CNT Printing

The 99.9% semiconducting CNT ink (IsoSol-S100, NanoIntegris) was purchased from

NanoIntegris with a concentration of 0.1 mg/mL. To print CNTs on Kapton, the ink was diluted

to a concentration of 0.05 mg/mL with toluene before printing. An Optomec AJ300 aerosol jet

printer with a 150 µm diameter nozzle was used to print CNTs. The printing speed was 8 mm/s,

and during the printing process, the platen temperature was kept at 23 °C (room temperature).

The sheath flow rate and the carrier gas flow rate were set to be 25 and 38 sccm, respectively,

and 330 mA ultrasonic atomizer current was applied. A single pass of CNT ink was printed for

CNT channels. To print CNTs on paper, the ink concentration, the printing speed, the sheath

flow rate, and the carrier gas flow rate were set to be 0.02 mg/mL, 2 mm/s, 40 sccm, and 23

sccm, respectively. The platen temperature, the ultrasonic atomizer current, and the number of

printing passes were the same as those for printing CNTs on Kapton.

In-Place and External Rinsing

For in-place rinsing, the platen temperature was first increased to 80 °C once the CNT

printing had been finished, and toluene was then applied onto the channel region by a squeeze

bottle. This rinsing step lasted for about 20 s and was followed by an immediate blow-drying

step using a nitrogen gun (see Supporting Video 1).

External rinsing was carried out in a fume hood. The channel region was first rinsed with

toluene by a squeeze bottle for about 30 seconds, and the sample was then immersed in 70 °C

toluene for another minute to further remove the polymer surfactant. After taking the sample out

from the hot toluene bath, another brief toluene rinse was carried out using the squeeze bottle,

and the sample was blow dried with nitrogen.

h-BN Ink Preparation and Characterization

h-BN ink was prepared according to the ultrasonic-assisted liquid phase exfoliation

protocol described in ref 41. In brief, bulk hexagonal boron nitride powder (Sigma-Aldrich, >1

µm, 98% grade) was dispersed to a concentration of ~3 mg mL-1 in DI water (resistivity 18.2 MΩ

cm), along with 1-pyrenesulfonic acid sodium salt (PS1) (Sigma-Aldrich, purity ≥ 97%)

dispersed to a concentration of 1 mg mL-1. After sonication for 120 h in a 300 W Hilsonic HS

1900/Hilsonic FMG 600 bath sonicator at 20 °C, the dispersion underwent a series of

centrifugation steps using a Sigma 1-14K refrigerated centrifuge. 42 The first centrifugation step

was performed at 3500 rpm for 20 min (g factor = 903), designed to separate out any bulk,

nonexfoliated flakes which may remain. Subsequently, three washing centrifugation steps were

performed, in which the excess PS1-containing solution was replaced with pure DI water.

The concentration of the h-BN dispersion was derived through UV-vis spectroscopy,

using a Varian Cary 5000 UV−vis spectrometer and the Beer−Lambert law, for which the

absorbance was taken at a wavelength of 550 nm and an extinction coefficient of 1000 L g-1 m-1

was used.58 Details of the lateral size and thickness characterization of the resultant h-BN flakes

are given in ref 42. In brief, most of the flakes are single and few layers and have lateral size

between 85 and 230 nm. The resulting dispersion has a concentration of 2 mg/mL, and 0.11

mg/mL HPMC (Sigma-Aldrich, 09963) and 5.7% v/v propylene glycol (99.5% Sigma-Aldrich)

were added into the dispersion to modify the rheological properties and to enhance substrate

adhesion.

h-BN Printing

Aerosol jet printing was done using an Optomec AJ300 aerosol jet printer. A 200 µm

diameter nozzle was utilized, and the platen temperature was set to be 80 °C. The sheath flow

rate and the carrier gas flow rate were set to be 25 and 40 sccm respectively, and the current

applied to the ultrasonic power was 390 mA. The h-BN film was printed with a platen speed of 2

mm/s; numbers of printing passes for thin films and capacitors varied, but three passes of h-BN

were always printed for CNT-TFTs, resulting in a film thickness around 2.54 µm. For each print,

1 mL of as-prepared ink was loaded into the atomizer, and after the printing was finished, the

samples were left on the 80 °C platen for 20 min to drive off the moisture in the network.

AgNW Printing

The AgNW ink preparation process is described in ref 27. The concentration of the

AgNW ink was 10 mg/mL, and 0.09 mg/mL HPMC and 3.8% v/v propylene glycol were added

into the ink to modify the ink rheology properties and improve the pattern morphology. AgNW

ink was printed using an Optomec AJ300 aerosol jet printer with the platen temperature set at 80

°C. A 200 µm diameter nozzle was utilized, and 25 sccm sheath flow rate and 35 sccm carrier

gas flow rate were used with the current of the ultrasonic atomizer set at 370 mA. The printing

speed was 2 mm·s-1. A single pass of AgNW was printed for the pads, and three to four passes

were printed for leads. For each print, 1 mL of as-prepared ink was loaded into the atomizer, and

after the printing had been finished, the samples were left on the 80 °C platen for 10 min to drive

off the moisture in the network.

Microscopy and Profilometry Characterization

Optical images were taken using a Zeiss Axio compound microscope; 2.5x and 10x

objective lenses were used in this study, and the images were captured via the incorporated

digital camera connected to a computer. Scanning electron microscopy imaging was performed

using an Apreo S SEM from ThermoFisher Scientific. An accelerating voltage of 2 kV was used,

and the emission current was set to be 25 pA. To reduce the charging effect from insulating

layers, patterns were printed on SiO2 (300 nm)/Si substrates, and when the cross-section of the

films was imaged, both sides of the chips were wrapped with copper tape. Atomic force

microscopy (Digital Instruments Dimension 3100) was utilized to study the morphology of CNT

film. Film thickness measurement was carried out using the Bruker Dektak 150 profilometer

with 2 mg force applied.

Characterizations of Electrical Performances

Electrical characterization of printed devices, including CNT-TFTs and capacitors, was

performed on a Lakeshore vacuum probe station interfaced with an Agilent B1500A

semiconductor parameter analyzer. The measurements were done under ambient pressure.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI:

10.1021/acsnano.9b04337.

Aerosol jet printer working mechanism; optical images illustrating the fabrication process; the

impact of ink formulation; capacitor fabrication; h-BN film thickness; impedance analysis; a

more rigorous estimation on gate capacitance; additional electrical characterizations; a

mechanical flexibility and process temperature comparison between 1D-2D TFTs and previously

reported printed CNT-TFTs (PDF)

A video file illustrating the in-place rinsing process (AVI)

AUTHOR INFORMATION

Corresponding Author

Email: [email protected] . Tel: +1-919-681-9471

Author Contributions

S.L., J.C., and A.F. designed the experiments. S.L., J.C., and J. A. fabricated the devices

and carried out the characterization. N.W. and R.W. synthesized the AgNW and h-BN

inks, respectively. A.F. supervised the work. S.L., R.W., C. C. and A.F. wrote the

manuscript with revision and approval from all authors.

ACKNOWLEDGMENT

S.L. and A.D.F. acknowledge support from a grant from the U.S. Army through the

congressionally directed medical research program under grant W81XWH-17-2-0045. N.W. and

A.D.F. acknowledge support from a grant from the National Institute of Health (NIH) under

grant 1R01HL146849. C.C. acknowledges the Grand Challenge EPSRC grant EP/ N010345/1,

the RFID SHP2 project in the framework of the graphene flagship (grant agreement No 785219)

and the European Union’s Horizon 2020 research and innovation programme under the ICT

project WASP (grant agreement No. 825213). R.W. acknowledges the Hewlett-Packard

Company for financial support in the framework of the Graphene NowNano Doctoral Training

Center. This work was performed in part at the Duke University Shared Materials

Instrumentation Facility (SMIF), which is a member of the North Carolina Research Triangle

Nanotechnology Network (RTNN), which is supported by the National Science Foundation

(Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure

(NNCI).

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