Black phosphorus (BP) is a new type of two-dimensional (2D) semiconducting material with a direct bandgap that changes with its thickness.^[1–5] Few-layer BP has excellent electrical transport properties, such as high carrier mobility (10⁴ cm² · V⁻¹ · s⁻¹) and large current on/off ratio (1 × 10^5[6–9] Additional, BP shows prominent in-plane anisotropic electrical characteristics, which origin from its honeycomb structure,^[10] quantum transport characteristics of carriers,^[11] and excellent optical-response properties.^[2,12–19] Because of these attractive properties, BP has been regarded as a promising material for applications in (photo) electronic devices, phototherapy, thermoelectric devices and sensors.^[20–41] However, the instability of BP under ambient conditions affects its practical applications.^[42–46] Previous study shows that metal ion can be modified on BP surface via the cation–π interaction to enhance both the ambient stability and transport properties of BP. Among the metal ions, Ag⁺ shows a relatively high binding energy with BP, which is 41.8 eV.^[47] Such a strong binding energy and the transport properties enhancement effect give BP-FET device a new application direction, Ag⁺ detection.

Ag⁺ ion, a heavy metal ion, has outstanding bactericidal performance because it can be combined with the sulfhydryl (–SH) in the bacteria, leading to the inactivation of the protease in the bacteria with –SH as an active unit. Of all the metal ions, Ag⁺ ion has the second highest bactericidal activity and Hg²⁺ ion has the highest bactericidal activity but it is toxic. Therefore, Ag⁺ ion is widely used as food functional additives. However, human protease activity can also be suppressed by high concentration of Ag⁺ ions, and excessive Ag⁺ ions entering into human body through food will seriously harm human health.^[48,49] Therefore, it is very important to detect and control the residual of Ag⁺ ions in food. Different detection methods will be used for Ag⁺ ions indifferent samples, such as atomic absorption spectrometry (AAS),^[50] inductively coupled plasma (ICP),^[51–53] etc. Although these methods are accurate enough for quantitatively analysing the Ag⁺ ions in the samples, high cost, time-consuming, and complicated instrumentations are the main factors hindering their applications for fast in-situ detection.

In this work, the detection of Ag⁺ in N-methyl pyrrolidone (NMP) solution is first realized by the BP-FET sensor through the cation–π interaction between Ag⁺ and BP. The detection limit for Ag⁺ by the BP-FET chip method is 10⁻¹⁰ mol/L(0.01 ppb), which is below the allowable limit concentration (0.1 ppm) of Ag⁺ ions in drinking water specified by the World Health Organization (WHO). And the detection time for each sample is 60 s. Finally, the mechanism of BP-FET sensor for Ag⁺ detection is investigated. Such a simple BP-FET sensor can provide an effective way for actual detection of Ag⁺, especially in witnessed inspections field of food.

Figure 1 is a schematic diagram of BP-FET sensor preparation and testing process. The device preparation process consists of five steps. Firstly, few-layer BP sheets were obtained by mechanical exfoliation with scotch tape and then were transferred onto the surface of Si/SiO₂ (300 nm) substrate by a PDMS thin film as a medium. Secondly, the sample thickness was determined by an optical microscope and an atomic force microscope. Thirdly, the sample on the Si/SiO₂ (300 nm) substrate was spin-coated with MMA and PMMA and dried by heating plate, respectively. Then, EBL, developing processes and electron beam evaporator of semiconductor processes were used to define the position and shape of drain and source electrodes. Finally, the BP-FET sensor was obtained after lift-off process with acetone whose typical electrodes were made up of chromium (5 nm in thickness) and gold (40 nm in thickness). The silicon substrate whose surface oxide was partially removed by the silicon blade was used as the back-gate electrode.

Sequentially, the device testing process consists of three steps. Firstly, BP-FET sensor was soaked in NMP solution containing Ag⁺ ions for different times and different concentrations. Then, the device was removed, washed with NMP, and dried with nitrogen rapidly. Finally, electrical properties of the BP- FET sensor were tested by Keithley 4200 semiconductor characteristic analyzer system combined with a probe station in air.

Fig. 1.

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	Fig. 1. (color online) Schematic diagram of fabrication, Ag+ adsorption and measurement process of BP-FET sensor.

Typical BP films each with a thickness of less than 30 nm are selected through an optical microscope by optical contrast identification. The exact thickness of BP sheets is determined by AFM. Figure 2(a) shows surface topographic image of a BP sheet obtained from AFM with standard tapping mode. Height image of BP sample as shown in Fig. 2(b), indicates that the thickness is about 10 nm. Figure 2(c) shows the Raman scattering spectra obtained from a BP sheet adsorbed Ag⁺ (BP_Ag(+)) and a pristine BP sheet, respectively. Each spectrum shows three characteristic peaks at ∼ 369 cm⁻¹, ∼ 438 cm⁻¹, and ∼ 466 cm⁻¹, which corresponds to the , B_2g, and vibrational modes of P in BP.^[23,54–56] The peak corresponds to the vibrational mode of out-of-plane, and B_2g and peaks correspond to the vibrational modes of in-plane. The selective enhancement of the peak (as shown in Fig. 2(c)) indicates that the Ag⁺ ions are successfully adsorbed on the surface of the BP sheet.

Fig. 2.

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	Fig. 2. (color online) (a) AFM topographic image of BP sheet. (b) AFM height profile of BP sheet obtained from panel (a), demonstrating a height of about 10 nm. (c) Raman spectra of BP sheet and BPAg(+) sheet.

Fig. 3.

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Fig. 3. (color online) (a)Ids versus Vds curves obtained from the BP-FET after Ag+ adsorption for 0 s, 20 s, 40 s, 60 s, 80 s. Vgs = 0 V. (b) Ids versus adsorption–time curves obtained from panel (a), at Vd = 0.2 V, 0.5 V, and 1.0 V, respectively. (c) Ids versus Vds curves obtained from the BP-FET after Ag+ adsorption for 60 s with different Ag+ concentrations ranging from 10−10 mol/L to 10−6 mol/L at room temperature, Vgs = 0 V. (d) Conductance versus ion concentrations curves obtained from panel (c), at Vds = 1.0 V, Vgs = 0 V (inset: logarithmic scale).

After the BP-FET sensor is prepared, the Ag⁺ adsorption time dependence and Ag⁺ adsorption concentration dependence of the device were studied by Keithley 4200 semiconductor characteristic analyzer system (Keithley, 4200SCS), and the results are shown in Fig. 3.

A BP-FET sensor is soaked in NMP solution for 0 s, 20 s, 40 s, 60 s, 80 s, separately, of which the Ag⁺ concentration is 10⁻¹⁰ mol/L. Then, the source–drain current I_ds versus source–drain voltage V_ds characteristics for different Ag⁺ adsorption time are obtained as shown in Fig. 3(a). The back-gate voltage V_gs is 0 V. The Ag⁺ adsorption time dependence current–time curves obtained from Fig. 3(a), with V_d = 0.2 V, 0.5 V, and 1.0 V, respectively,indicate that the current increases with the adsorption time of Ag⁺ ions increasing. After the adsorption time is greater than 60 s, the increasing current becomes saturated as shown in Fig. 3(b). It implies that the detection time is only 60 s. However, the thickness of the BP directly affects the sensing performance of the device. As shown in Fig. S5 in Appendix A, the sensing characteristics of the device on thin BP (3 nm–5 nm) are fairly good, but its own performance is unstable, which is not suitable for accurately Ag⁺ detection. For the devices on thick BP (∼ 20 nm), the sensing performance is not good. In this case, we choose the BP with a thickness of around 10 nm for the FET device fabrication.

After BP-FET sensor is soaked in NMP solution for 60 s, with different Ag⁺ ion concentrations ranging from 10⁻¹⁰ mol/L to 10⁻⁶ mol/L at room temperature, the source–drain current I_ds versus source–drain voltage V_ds characteristics are investigated as shown in Fig. 3(c). The back-gate voltage V_gs is set to be 0 V. The Ag⁺ ion concentration-dependent current curves obtained from Fig. 3(c), with V_d = 1.0 V, indicates that the conductance increases with the concentration of Ag⁺ ions increasing. After the concentration of Ag⁺ ions is larger than 10⁻⁶ mol/L, the increasing conductance becomes saturated as shown in Fig. 3(d) (inset: logarithmic scale). It implies that the concentration detection capability of BP-FET sensor for Ag⁺ ions ranges from 10⁻¹⁰ mol/L to 10⁻⁶ mol/L. For the Ag⁺ sample with a concentration of 10⁻¹¹ mol/L, the device cannot well work as a sensing medium, even the adsorption time for Ag⁺ is extended to 10 min as shown in Fig. S6 in Appendix A.

The sensing mechanism of BP-FET sensor is investigated systematically. The conductance of BP can be expressed as the following formula:

Figure 4(a) is the output curve of the BP-FET device before and after Ag⁺ adsorption, in which the adsorption time is 60 s and the Ag⁺ concentration is 10⁻¹⁰ mol/L at V_gs = 0 V. Figure 4(b) shows the transfer curve of the BP-FET device before and after Ag⁺ adsorption under the same conditions, at V_ds = 0.5 V. The channel length and width of the device are typically 3 μm and 3 μm, respectively. So, according to the formula of carrier mobility^[1]

Fig. 4.

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	Fig. 4. (color online) (a) Output and (b) transfer curves of BP-FET sensor before (black) and after (red) Ag+ adsorption (10−10 mol/L, 60 s), with channel length and width of the device being both 3 μm.

We continue to explore the process of Ag⁺ adsorption on the BP surface. Concretely, Ag⁺ ion interacts with the conjugated π bond, which derives from the lone pair electrons of the phosphorus atoms in surface layer, via the cation–π interaction, as shown in Fig. 5(a).^[47] BP is a p-type semiconductor with adjustable band gap, so its majority carriers are holes and its Fermi energy is close to the conduction band. The Ag⁺ ion adsorption acts as a negative back-gate voltage on the surface of BP sheet because of the positive characteristic of Ag⁺ ions, which will inject additional holes into BP sheet. As a result, Fermi level (E_F) of BP sheet will move close to the valence band as shown in Fig. 5(b), which will increase hole concentration in BP sheet and then increase its conductance.Beside Ag⁺ ions, other kinds of metal ions like Mg²⁺ and Fe³⁺ can also be adsorbed on the surface of BP working as an additional hole medium and enhance the p-type transport performance of the devices as shown in Fig. S7 in Appendix A.

Fig. 5.

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	Fig. 5. (color online) (a) Schematic of Ag+ adsorption to the surface of BP-FET. (b) Band structure of BP before and after Ag+ absorption. It shows that Fermi level of BP sheets moves close to valence band, increasing hole concentration and conductance of BP sheet because of Ag+ absorption.

In this research, a high-performance BP-FET sensor for Ag⁺ detection with high sensitivity, rapid detection speed and wide concentration detection range has been fabricated by micro-nano machining. Based on the BP-FET sensor, the detection limit for Ag⁺ can reach 10⁻¹⁰ mol/L (∼ 0.01 ppb), which is far below the allowable limit concentration (0.1 ppm) of Ag⁺ in drinking water. The detection time is 1 min for each sample. The reason why BP-FET can be used for Ag⁺ detection is the cation–π interactions between Ag⁺ and BP. When Ag⁺ ions attach on the surface of BP, which equivalent to introducing plenty of holes into a bipolar semiconductor, the p-type transport behavior of the device will be enhanced. The BP-FET sensor shows a new application direction of FETs and provides an effective method for the actual detection of Ag⁺, especially in witnessed inspections field of food.