Ink formulation of functional nanowires with hyperbranched stabilizers for versatile printing of flexible electronics

Materials

Hydroxyl-ended hyperbranched polyester (HBP-2) was provided by Wuhan Hyperbranched Polymers Science & Technology Co., Ltd., China. 3-mercaptopropionic acid (MPA, ≥ 98%), p-toluenesulfonic acid (TsOH, ≥ 98.5%), anhydrous sodium sulfate (≥ 99%), sodium hydroxide (NaOH, ≥ 96%), sodium chloride (NaCl, ≥ 99.5%), PVP (K88-96), toluene (TL), acetone (ACE, ≥ 99.5%), ethanol (EtOH, ≥ 99.5%), isopropanol (IPA, ≥ 99.7%), DMF (≥ 99.5%), DMSO (PharmPure™), and NMP ( ≥ 99%) were purchased from Aladdin Chemical Reagent Co., Ltd., China. Diethylenetriamine (DETA, > 99%), methyl acrylate (MA, ≥ 99%), and ethylenediamine (EDA, ≥ 99%) were supplied by Sinopharm Chemical Reagent Co., Ltd. Ethylene glycol (EG, > 99%) was purchased from Concord Co., Ltd., China. Silver nitrate (AgNO₃, ≥ 99.8%) was obtained from Beijing InnoChem Science & Technology Co., Ltd., China. Sodium tellurite (Na₂TeO₃, 99%) and L-ascorbic acid (≥ 99%) were purchased from Sigma-Aldrich. The filtration membrane (8 µm average pore size) was purchased from Zhongli Filter Equipment Factory Co., Ltd., China. PET film (thickness 0.125 mm) was purchased from Shenzhen Tenglong Packaging Materials Co., Ltd., China. Copper nanowires (CuNWs, length ≈5 μm and diameter ≈200 nm) were purchased from Nangong Bole Metal Material Co., Ltd., China. Zinc oxide nanowires (ZnONWs, length ≈5 μm and diameter ≈50 nm) were purchased from Xuzhou Jiechuang Material Technology Co., Ltd., China. The PDMS mixture (SYLGARD 184) was purchased from Dow Corning Co., Ltd., China. All reagents were of analytical grade and used as received.

Characterization

The morphology of nanowires was imaged using a JSM-7800F field-emission scanning electron microscope (JEOL, Japan). TEM analysis was performed on a JEM-2800 transmission electron microscope (JEOL, Japan). XPS was conducted on a Thermo Scientific K_α X-ray photoelectron spectrometer (ThermoFisher, USA). The crystallinity of the nanowires was characterized by powder X-ray diffraction (Rigaku D/Max-2500, Japan). Optical microscopy images were obtained using an upright metallurgical microscope (Leica DM750 M, Germany). Digital photos of the samples were acquired using a Canon 5D Mark III camera. Optical transmission spectra of NHPM-AgNWs films were obtained using a UV-2600 spectrometer (Shimadzu, Japan). The sheet resistance of NHPM-AgNWs films was measured on an ST-2258C four-probe instrument (Suzhou Jingge Electronic Co., Ltd., China). The rheological properties of the nanowire inks were measured using a DHR-2 rheometer (TA Instruments, USA) at room temperature (25.0 ± 0.3 °C). The steady-state flow step (SSFS) test was used to measure the shear viscosity of the inks at shear rates in the range of 0.1–1000 s⁻¹. The peak hold step (PHS) test was performed with constant shear rates in three intervals (0.1 s⁻¹ shear rate for 30 s, 200 s⁻¹ for 30 s, and 0.1 s⁻¹ for 60 s). The stress sweep step (SSS) test was performed with an oscillation stress range of 1–1000 Pa at a frequency of 1 Hz. The temperature and temperature distribution of sensors were measured using K-type thermocouples and infrared thermal imagers (Testo 869, Germany), respectively. Laser etching was performed on an LPKF ProtoLaser R4 (LPKF, Germany) with a pulsed laser (λ = 515 nm) at a repetition rate of 100 Hz. All measurements were carried out at room temperature.

FoM calculation

The FoM is a representative quantity used to evaluate the performance of transparent electrodes, and could be expressed as:

where σ_dc is the direct current conductivity of the film, σ_op (λ) is the optical conductivity at wavelength of λ (550 nm), R_s is the sheet resistance, and T is the transmittance at λ (550 nm)⁴⁰.

Electrical conductivity measurements

The electrical conductivity (ρ) of the as-prepared patterns was calculated using Eq. (2).

where L is the length of the as-prepared patterns, R is the electrical resistance, and S is the cross-section area of each pattern measured from the SEM images.

Resistance-change measurements

The electrical response of the slot-die coated AgNW electrode was measured by subjecting it to cyclic bending with a bending angle of 180° for 10,000 bending-unbending cycles. A motorized linear stage with a built-in controller (Zaber Technologies) was used to bend the samples by varying the distance of two stepper motors. Changes in the resistance of the samples were recorded using a digital multisource meter (Keithley 2400, Tektronix).

Stretching tests of 3D-printed flexible serpentine-patterned electrodes were performed using a motorized linear stage with a built-in controller (Zaber Technologies) by varying the distance between the two stepper motors. Changes in the resistance of the samples were recorded using a digital multisource meter (Keithley 2400, Tektronix).

Temperature sensing measurements

To conduct temperature-variable sensing measurements, a TeNW-based temperature sensor was first placed on a temperature-control module calibrated using a Peltier module to precisely control the temperature. Throughout this procedure, a Keithley 2400 digital multisource meter recorded changes in the resistance of the TeNW-based temperature sensors at different temperatures. The performance of the TeNW-based temperature sensing device was evaluated by calculating the TCR using Eq. (3).

where T and ΔR/R₀ refers to the temperature and relative resistance change, respectively⁶⁶.

Optoelectronic sensing measurements

The optoelectronic properties of the photodetectors were measured using a digital multisource meter (Keithley 2400). A 365 nm UV lamp was used as the UV source, and the optical power intensity was varied using a combination of neutral density filters. The photocurrent was measured at 1 V under UV illumination. The photocurrent on-off ratio was calculated by Eqs. (4) and (5):

where I_light and I_dark are the current when the UV illumination was switched on and off, respectively.

The responsivity was calculated using Eq. (6).

where R, P, and A are the responsivity, incident light power density, and effective area of an individual pixel, respectively⁶⁷. All measurements were performed at room temperature under ambient conditions.

Computational studies

Vienna Ab-inito Simulation Package (VASP) was used to conduct all DFT calculations^68,69. The Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was used to describe the exchange-correlation effects^70,71. The core-valence interactions were accounted for by using the projected augmented wave (PAW) method⁷². The energy cutoff for plane wave expansions was set to 400 eV, and 3 × 3 × 1 Monkhorst-Pack grid k-points were selected to sample the Brillouin zone integration. Structural optimization was completed for an energy and force convergence set at 1.0 × 10⁻⁴ eV and 0.01 eV Å⁻¹, respectively.

The binding energy (E_b) was defined as:

where the slab and cluster subscripts refer to the metal surface and adsorbate molecule, respectively.

NHPM synthesis

DETA (0.30 mol) and MA (0.30 mol) were added to a 250 mL three-necked flask equipped with a mechanical stirrer, a nitrogen inlet, and a water trap attached with a condenser²⁴. The mixture was mechanically stirred at 0–5 °C for about 12 h, and then 0.05 mol EDA was dripped into the flask. The mixture was slowly heated to 70 °C and allowed to react for 1 h and then heated to 130 °C and reacted for an additional 7 h under a nitrogen atmosphere. The yield was about 96%, resulting in a yellowish liquid product named NHPM. Its theoretical molecular weight was calculated to be 1002 g mol⁻¹ according to its molar ratio.

SHPM synthesis

SHPM was synthesized via the esterification of MPA and HBP-2²⁵. HBP-2 (0.03 mol), 0.43 mol MPA, 1.62 g p-toluenesulfonic acid, and 75 mL toluene were added consecutively to a three-necked flask equipped with a mechanical stirrer, a nitrogen inlet, and a water trap attached with a condenser. The reaction was carried out for 12 h at 80 °C. After the reaction mixture was cooled to room temperature, the obtained solution was washed with 5 wt.% NaOH (3 × 7.5 mL). The organic layer was dried with anhydrous sodium sulfate, filtered, and then distilled under reduced pressure. The yellow liquid product was obtained with a yield of about 93%. The product was named SHPM, and its theoretical molecular weight was calculated to be 2260 g mol⁻¹ according to its molar ratio.

AgNW synthesis

AgNWs with an average length of 40 μm and diameter of 40 nm were synthesized via the polyol method reported in ref. ²⁸. PVP and ethylene glycol (160 mL, 12.5 mg mL⁻¹) were added to a brown conical flask, heated at 150 °C, and then stirred until PVP was completely dissolved. The solution was cooled to room temperature, and then sodium chloride in ethylene glycol (16 mL, 0.42 mg mL⁻¹) and silver nitrate in ethylene glycol (40 mL, 50 mg mL⁻¹) were successively added to the solution. After vigorous magnetic stirring for 5 min, the brown conical flask was transferred to an oven preheated to 110 °C for 12 h. The as-prepared AgNWs mixture was diluted with ethanol to 0.5 mg mL⁻¹. The 6 L diluted mixture was poured into a filtration membrane-based cylindrical chamber with a pore size of 8 µm and then mechanically stirred at 200 rpm²⁸. Ethanol slowly flowed continuously into the filtration setup, and the ethanol purification process was stopped after 80 min. While stirring and washing, the solution was concentrated to 1 mg mL⁻¹, and the purified AgNWs solution was collected.

TeNW synthesis

NHPM (0.549 mmol) was ultrasonically dissolved in 160 mL deionized water, and then 22.7 mmol L-ascorbic acid was added and magnetically stirred until a clear solution was formed. Na₂TeO₃ (0.939 mmol) was added to the homogeneous solution, and the obtained suspension was continuously heated and stirred at 90 °C for 24 h. The crude reaction mixture was cooled to room temperature and centrifuged successively with deionized water and ethanol until the supernatant was transparent. The obtained precipitates were ultrasonically dispersed in 15 mL of distilled water and then freeze-dried to obtain tellurium nanowires (NHPM-TeNWs) with an average length of 1.5 μm and diameter of 50 nm.

NHPM-AgNW inks formulation

AgNW dispersed in ethanol (1 mg mL⁻¹) were coordinated with the NHPM dispersant at an optimal weight ratio of 1:0.001 and then centrifuged. Less amount of HPMs would affect the dispersibility of AgNWs in the inks. More HPMs would decrease the conductivity of the printed patterns. The precipitates were dispersed in different solvents with different nanowire concentrations to prepare NHPM-AgNW inks.

SHPM-AgNW inks formulation

AgNW dispersed in ethanol (1 mg mL⁻¹) were coordinated with the SHPM dispersant at a weight ratio of 1:0.001 and then centrifuged. The precipitates were dispersed in different solvents with various nanowire concentrations to prepare SHPM-AgNW inks.

NHPM-TeNW inks formulation

TeNW dispersed in IPA (0.5 mg mL⁻¹) were mixed with the NHPM dispersant at a weight ratio of 1:0.001 and then centrifuged. The precipitates were dispersed in different solvents with various nanowire concentrations to prepare NHPM-TeNW inks.

NHPM-ZnONW inks formulation

ZnONW dispersed in deionized water (0.5 mg mL⁻¹) were mixed with the NHPM dispersant at a weight ratio of 1:0.001 and then centrifuged. The precipitates were dispersed in different solvents with various nanowire concentrations to prepare NHPM-ZnONW inks.

NHPM-CuNW inks formulation

CuNW dispersed in ethanol (1 mg mL⁻¹) were coordinated with the NHPM dispersant at a weight ratio of 1:0.001 and then centrifuged. The precipitates were dispersed in different solvents with various nanowire concentrations to prepare NHPM-CuNW inks.

Fabrication of AgNW transparent electrode

PET films were secured to an automatic coater equipped with a Meyer rod (RDS 6). Then, NHPM-AgNW ink was pipetted at the top of the film, and the Meyer rod was quickly drawn down over the NHPM-AgNW ink to spread it across the PET into a thin, uniform film. Different densities of NHPM-AgNW were formed on the substrate surface by controlling the nanowire concentration. The films coated with NHPM-AgNWs were dried in the air for 5 min at 80 °C without any additional harsh posttreatment process to optimize their conductivity. Washing and annealing (at temperature higher than 120 °C) the as-printed AgNWs would not further improve their conductivity.

Slot-die coating nanowire inks and laser etching

NHPM-AgNW IPA ink (solid content of 7.1 wt.%) was printed onto a paper substrate by slot-die coating and dried in the air for 5 min at 80 °C to form a conducting layer. The optoelectronic sensing device was produced using an LPKF ProtoLaser R4 (LPKF Laser & Electronics AG, power = 8 W) with a pulsed laser (λ = 515 nm) at a repetition rate of 100 Hz. The etching width and depth were varied by modulating the laser spot size and pulse intensity. Ten pairs of interdigitated electrodes were prepared, with 19 interspaces between adjacent electrodes and a fixed interspace between interdigitated AgNWs lines of 30 μm. The width and length of the interdigitated AgNWs lines were 60 μm and 1400 μm, respectively. The as-prepared NHPM-ZnONW IPA ink (solid content of 80 mg mL⁻¹) was screen-printed onto the interdigitated electrodes to finish the construction of ultraviolet photodetectors.

Screen printing nanowire inks

Screen-printing tests were performed on a screen printer (TC-4060k, Dongguan Ta Chen Screen Printing Machine & Materials Co., Ltd.) using a precision stainless-steel screen mesh (400 mesh count, Dongguan XiangPeng Screen Printing Equipment Co., Ltd.). A squeegee with an ergonomic holder formed an angle of ≈45° with the screen mesh. The printing speed was ≈100 mm s⁻¹, and the printing force was ≈32.2 N. NHPM-AgNW IPA ink (solid content of 13.2 wt.%) was used to print patterns. The printed patterns were dried in the air for 5 min at 80 °C.

Extrusion-based 3D printing nanowire inks

Serpentine patterns and coils were prepared by extrusion-based 3D printing using a pre-programmed benchtop robot. NHPM-AgNW IPA ink (solid content of 20.3 wt.%) was extruded using an air-powered fluid dispenser (FiSNAR, DC 100) with a needle diameter of 250 μm, a pressure of 15.0 bar, and a movement speed of 3 mm s⁻¹. The liquid PDMS mixture (SYLGARD 184, Dow Corning, with a weight ratio of silicone elastomer to curing agent of 10 to 1) was coated on a glass slide and cured at 80 °C for 60 min to obtain a layer that was 300 μm thick. Then, a serpentine pattern was printed on a PDMS film, and the electrode was encapsulated by PDMS using a drop-casting method and then dried at 70 °C for 1 h.

NFC tag fabrication

An NFC antenna was screen printed on a paper substrate (length ≈6 cm and width ≈5 cm) using the NHPM-AgNW IPA ink (solid content of 13.2 wt.%), and then a jumper resistor, LEDs, and chip (IC-CUID, ISO 1443) were welded to obtain AgNW NFC tags. The printed antenna was composed of 10-turn coils with a line width of 250 μm and a coil spacing of 250 μm. Using the NFC function of a mobile phone, the access card program, personal information, and application package were written into the AgNW NFC tags, respectively. The AgNW NFC tags were used as an access card for standard electronic door locks, an identification label for personal information, and to control a smartphone app.

Fabrication of sensory e-textile

A TeNW-based temperature sensor was screen printed, and the printing speed, force, and angle between the stencil and squeegee were optimized for the NHPM-TeNW H₂O ink (solid content of 16.7 wt.%). The conductive electrodes with a trace width and spacing of 1.0 mm and 1.8 cm were screen printed using the NHPM-AgNW IPA ink (solid content of 13.2 wt.%) on the surface of polyester fabric. The pattern (length ≈2.5 cm and width ≈1 cm) of the screen-printing plate was installed in the screen printer, and then the NHPM-TeNW H₂O ink was applied onto the screen-printing plate and printed onto the polyester fabric by sliding the squeegee over the stencil. The printed device was dried at 60 °C for 5 min to evaporate the water before subsequent tests.

The wearable e-textile was composed of screen-printed AgNW circuits, a TeNW-based temperature sensing device, a flexible printed circuit board (FPCB, with a built-in Bluetooth module), and a polyester fabric substrate. The wearable e-textile integrated a microcontroller unit (MCU) and battery management system. The MCU possessed a built-in Bluetooth module and an analog-to-digital converter (ADC). The battery supplied power to the wearable e-textile using a battery management system. To collect the resistance signal, the wearable e-textile provided an excitation signal to the TeNW-based temperature sensor. The ADC collected the voltage signal from the TeNW-based temperature sensor and then converted it into a resistance signal through an internal program of the chip (ESP32-PICO-D4, Espressif Systems (Shanghai) Pte., Ltd., China). This was performed according to the principle of resistance voltage division, and then data analysis and calculations were performed. Subsequently, the temperature data were wirelessly transmitted to the smartphone application via the Bluetooth module.

Fabrication of photodetector array

A photodetector array (10 × 10 pixels) on a paper substrate (5 × 5 cm²) was fabricated using AgNWs as interdigitated electrodes and ZnONWs as the active layer. The interdigitated electrodes were fabricated by slot-die coating the NHPM-AgNW IPA ink (solid content of 7.1 wt.%) on paper, followed by laser etching. Five pairs of interdigitated electrodes were prepared, with nine interspaces between adjacent electrodes. The interspace between interdigitated AgNWs lines was controlled at 60 μm, and the width and length of the interdigitated AgNWs lines were 60 μm and 800 μm, respectively. Then, the as-prepared NHPM-ZnONW IPA ink (solid content of 9.2 wt.%) was screen-printed onto the interdigitated electrodes to finalize the construction of the ultraviolet photodetector array.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.