Air-Stable Black Phosphorus Devices for Ion Sensing - ACS Applied

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Air-Stable Black Phosphorus Devices for Ion Sensing Peng Li, Dongzhi Zhang, Jingjing Liu, Hongyan Chang, Yan’e Sun, and Nailiang Yin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

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ACS Applied Materials & Interfaces

Air-Stable Black Phosphorus Devices for Ion Sensing Peng Li1*, Dongzhi Zhang2, Jingjing Liu2, Hongyan Chang2, Yan’e Sun2, Nailiang Yin2 1

State Key Laboratory of Precision Measurement Technology and Instruments, Department of

Precision Instruments, Tsinghua University, Beijing 100084, China. 2

College of Information and Control Engineering, China University of Petroleum (East

China), Qingdao 266580, China

ABSTRACT: Black phosphorus (BP) is one of the most attractive graphene analogues, and its properties make it a promising nanomaterial for chemical sensing.

However, mono-

and few-layer BP flakes are reported to chemically degrade rapidly upon exposure to ambient conditions.

Therefore, little is known about the performance and sensing mechanism of

intrinsic BP, and chemical sensing of intrinsic BP with acceptable air stability remains only theoretically explored.

Here, we experimentally demonstrated the first air-stable

high-performance BP sensor using ionophore coating. demonstrated significantly improved air stability.

Ionophore-encapsulated BP

Its performance and sensing mechanism

for trace ion detection were systematically investigated.

The BP sensors were able to realize

multiplex ion detection with superb selectivity, and sensitive to Pb2+ down to 1 ppb. Additionally, the time constant for ion adsorption extracted was only 5 s.

The detection

limit and response rate of BP were both superior to those of graphene based sensors. Moreover, heavy metal ions can be effectively detected over a wide range of concentration with BP conductance change following the Langmuir isotherm for molecules adsorption on surface.

The simplicity of this ionophore-encapsulate approach provides a route for

achieving air-stable intrinsic black phosphorus sensors that may stimulate further fundamental researches and potential applications.

KEYWORDS: black phosphorus, chemical sensing, air stability, ionophore, detection limit

ACS Paragon Plus Environment


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Black phosphorus (BP) is a layered material in which atomic thick layers are stacked together by van der Waals force1-4.

Mono- and few-layer BP shows both higher carrier

mobility5-7 (1,000 cm2·V−1·s−1) than that of transitional metal dichalcogenides (TMDs) field-effect transistors (FETs), including MoS2

8-10

, and larger current on/off ratio1,

11-13

(103–105) than that of graphene transistors14-16. Additionally, molecule adsorption energies of BP are larger than those of graphene and MoS217.

These properties, together with its

extremely large surface-to-volume ratio, make BP a promising nanomaterial for chemical sensing.

However, little is known about this material’s intrinsic sensing mechanism and

performance due to its fundamental challenge: lacking of air stability18-20.

Without

protection, BP samples start to degrade to oxygenated phosphorus (POX) shortly in ambient environment, with degradation of their electric properties and device performances21. results in a severe problem for device practical applications.

This

Abbas et al demonstrated gas

sensors based on thick BP flakes (55 nm) in order to reduce the influence of degradation22. However, thinner flakes may theoretically display better sensing performance due to larger surface-to-volume ratio.

Moreover, the existence of POx hindered exploring the sensing

mechanism and performance of intrinsic BP. BP flakes were reported to show improved stability after being encapsulated by dielectric layer, including Al2O3, which isolates the material from ambient environment21, 23.

Nevertheless, this approach is not suitable for

chemical sensing since the dielectric protection layer cuts off the connection between BP and molecules detected.

As such, to the best of our knowledge, chemical sensing of intrinsic BP

with acceptable air stability remains only theoretically explored. In this letter, we experimentally demonstrate the first air-stable BP sensor which is encapsulated with ionophore.

Ionophore film effectively reduces negative factors from

ambient environment, meanwhile, it allows certain type of molecules to selectively permeate through it24, 25.

As a result, high performance BP sensors with significantly improved air

stability were realized.

Their mechanism and performance for trace ion detection were

systematically investigated.

Trace ion elements of lead (Pb2+), cadmium (Cd2+), arsenite

(AsO2-), and mercury (Hg2+) can cause numbers of health problem, and are probable carcinogens for human beings with concentrations at ppb levels.

Therefore, detection of

these heavy metal ions is extremely important. Detection limit of BP sensor for Pb2+, Cd2+,


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AsO2-, and Hg2+ can reach 1 ppb, 3 ppb, 10 ppb and 1 ppb, respectively, within the permissible limits of the guideline for drinking water quality provided by the World Health Organization (WHO). The detection limit and response rate of BP are both better than those of graphene based sensors.

Our approach for enhancing BP sensor’s air stability can

stimulate further fundamental study on the interaction between intrinsic BP surface and molecules, and lead to various practical sensing applications. Multilayer BP flakes were mechanically exfoliated by scotch tape based method and transferred onto 300 nm thick SiO2 layer on low resistance Si substrates (Figure 1a).

BP

flakes of thickness less than 50 nm were initially chosen by optical microscope, using optical contrast identification approach. force microscopy (AFM).

The thickness was then identified accurately by atomic

Figure 1b is AFM tapping mode topographic image of a BP flake

with thickness of 12 nm (demonstrated in Figure 1c).

Raman spectra of mechanical

exfoliated flakes show three characteristic peaks (Figure 1d), which are the Raman shifts attributed to the A1g, B2g and A2g phonon modes (A1g at ~362 cm-1, B2g at ~440 cm-1, and A2g at ~466 cm-1).

The B2g and A2g peaks correspond to atoms vibrate within the plane, while

A1g peak corresponds to the oscillate out-of-plane20. sensors with a back-gate electrode.

We fabricated multilayer BP FET

The source/drain metal electrode fabrication process

involved photolithography, Ti/Pd (typically 7 nm and 90 nm) sputtering, and metal lift-off. The gap between source and drain electrodes was 5 µm.

Sequentially, 20 µL lead ionophore

solution was spun coated on BP surface at 5000 rpm for 60 s.

Samples were kept in air at

room temperature for 10-15 minutes in order to let the solvent, tetrahydrofuran (THF), evaporate and form a solid ionophore protection layer (Figure 1e). After device fabrication, their air stabilities were investigated.

BP samples

encapsulated and unencapsulated with ionophore were exposed to identical in-door ambient conditions (temperature 32-36 degree Celsius, relatively humidity 55-60%).

Bubble-like

features started to appear on the bare BP surface within 24 hours and they kept growing (Figure 2a-d, and supporting information).

AFM measurements demonstrated that the

height of the bubble can reach 700 nm after 4 days (see supporting information). Interestingly, bubbles were not evenly distributed on BP surface, and they were observed to



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gather around metal electrodes to form larger ones.

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In addition to the topographic feature,

the color of the flakes also changed (from pink to green), indicative of gradually etching and thinning of the flake. Thinner parts (less than 20 nm) were etched away after only 4 days exposure to air (see supporting information).

The electric properties of BP degraded

dramatically as well. After 24 hours, source-drain current IDS fell rapidly to 1/3 of its original value, approximately, and further reduced to only 1/7 on the third day (Figure 2g) due to both the flake thickness reduction and the formation of POX which conductivity is inferior to BP.

The main reason to cause degradation is the mixture of H2O and O2.

The

hydrophilic property of BP accelerates the water molecule absorption and promotes POX formation23.

Bubble-like features did not appear under metal electrodes, indicating that BP

can be passivated by coatings.

Ionophore film is able to effectively reduce the contact

between BP flakes and water molecules.

As such, capped samples showed no obvious

optical evidence for degradation for 30 days (no obvious bubbles or color change) (Figure 2e, f).

The ionophore-encapsulated BP devices were still in good shape after 1 week of ambient

exposure, with source-drain current IDS variation less than 10% (gate voltage VGS varied from -5 V to 10 V, source-drain voltage VDS=-2.5 V, as shown in Figure 2h), demonstrating significantly improved air stability.

Ionophore is able to reduce negative factors from

ambient environment, meanwhile, it allows certain types of ions to selectively permeate through it.

Hence, BP ion sensors with improved air stability can be realized.

Ion concentration dependence of BP sensors capped with lead ionophore was investigated and measured by semiconductor parameter analyzer (B1500A, Agilent Inc.). After introduction of sodium acetate buffer solution (0.1 M, pH=4.6) with various Pb2+ ion concentrations ranging from 0 to 100,000 ppb at room temperature, the source-drain current IDS versus gate voltage VGS characteristics were measured, as shown in Fig. 3a. source-drain voltage VDS was set at a constant value of 3 V.

The

As the Pb2+ ion concentration

increased, a clear reduction of IDS was observed, suggesting a reduction of the BP flake’s conductivity.

The material’s conductivity can be expressed by the equation below: =

(1)

where σ is conductivity, n is carrier density, q is charge per carrier, and µ is carrier mobility.


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Both carrier mobility µ and carrier density n may be responsible for the change of conductivity.

According to equation26: =

∆

(2)

∆

where L and W are FET length and width, respectively, is gate capacitance, carrier mobility µ is in proportion to

∆ ∆

.

The slope of IDS vs VGS curves in Figure 3a show

negligible variation with different Pb2+ ion concentrations (from -1.47 nA/V to -1.45 nA/V), indicating carrier density change instead of mobility change is the major reason for the reduction of BP conductivity during Pb2+ ion sensing.

We investigated the dependence of

sensing performance on the source-drain voltage VDS (Figure 3a-f).

When VDS increased in

either positive or negative direction, BP FET had more prominent source-drain current change with different Pb2+ concentrations, implying enhanced resolution.

Thus the

resolution of BP sensors is tunable, and larger VDS is desired for better sensing performance. Nevertheless, according to our experimental results, large VDS (10 V or above) may cause failure of BP device, inasmuch as the large source drain current IDS generated could damage the BP flakes or metal electrodes (see supporting information). Detection limit of a sensor is generally defined as three times the standard deviation of its noise 27 (defined by International union of pure and applied chemistry (IUPAC)).

The

output signal ∆IDS of BP sensor for 1 ppb Pb2+ was 3.3 nA, which was larger than three times the standard deviation of IDS noise (approximately 0.34 nA) (Figure 4a).

Therefore, the

detection limit of BP sensor for Pb2+ ion is capable of reaching 1 ppb (≈0.48×10-8 M), superior to graphene based Pb2+ detectors28 (2.51×10-8 M).

Figure 4b demonstrates

relative resistance change (∆R/R0) of BP versus time (VGS =0 V). We fit the curve with equation:

= + −

(3)

where r is instantaneous relative resistance change ∆R/R0, is the initial r after Pb2+ ion introduction, is the final r when the output signal is stable, t is the time, and is time constant.

The extracted time constant is only 5 s for a concentration of 100 ppb as shown



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in the inset of Figure 4b, better than graphene based ion sensors reported29, 30 (10 s and 15 s, respectively).

This fast response rate indicates the equilibrium between absorption and

desorption of ions on BP surface was established very quickly.

Relative resistance change

of BP is shown as a function of Pb2+ concentration in Figure 4c (experimental results from 7 Pb2+ ions can be effectively detected over a wide concentration range, from 10

sensors).

ppb to 100,000 ppb.

Resistance of BP increased linearly at low concentrations and showed This result implies that the adsorption of Pb2+ onto BP

saturation at higher concentrations.

surface followed the Langmuir isotherm31:

∆

=∆

!"#

+∆

%$(4)

!"#

where C is molecule concentration, Kd is the dissociation constant of the interaction between molecules and BP, ∆G is conductance change, and ∆GMax is the conductance change at saturation.

Our experimental results demonstrated good linear relation between

∆

and C,

fitting the Langmuir adsorption isotherm for molecule absorbed on surface well, as shown in the inset of Figure 4c. sensor.

As such, Langmuir adsorption model is applicable for BP chemical

This result further proves that the conductance change of BP flake was caused by

molecule absorption on BP surface. The sensing mechanism of black phosphorus was investigated.

BP is a p-type

semiconductor with moderate band gap1 (see supporting information), so its Fermi energy is close to the valence band, and holes are majority carriers. acts as a filter.

Lead ionophore film on top of BP

Theoretically, it only allows Pb2+ to pass through it, and blocks all the other

ions and molecules (Figure 5a).

The Pb2+ ions pass through ionophore and serve as a

positive gate voltage on top of BP, which repels positively charged holes.

As a result, Fermi

energy (EF) moves far away from the valence band (Figure 5b), which reduces hole density in material, with consequent reduction of conductivity.

On the other hand, absorption of

negative charged ions, including AsO2-, is predicted to attract more holes, and Fermi energy should move toward valence band, resulting better conductivity. We introduced buffer solution with different AsO2- , Cd2+, or Hg2+ concentrations to BP sensors covered with arsenite, cadmium, or mercury ionophore, respectively.


Figure 6a,

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b, and c demonstrate that the absorption of AsO2- clearly increased the source-drain current IDS, while Cd2+ and Hg2+ reduced the conductivity of BP.

The experimental results fitted

Additionally, BP sensors were sensitive to AsO2-, Cd2+ and Hg2+down

our assumption well.

to 10 ppb, 3 ppb, and 1 ppb, respectively.

Selectivity is critical for chemical sensors to

identify each type of molecule correctly and reduce the chance of misleading.

To verify the

ion selectivity of ionophore-protected BP sensors, AsO2-, Cd2+ and Hg2+ were introduced to BP sensors capped with lead ionophore.

Compared with the experimental results displayed

in Fig 6a, b, and c, their output signals, ∆IDS, were apparently smaller (Figure 6d, e, f).

BP

capped with other types of ionophores had similar response, indicating good selectivity of our BP sensors. In conclusion, this work is the first demonstration of air-stable high-performance BP chemical sensors.

After encapsulated with ionophore, BP sensors showed significantly

improved air stability.

The performance and mechanism of BP sensor for trace ion

detection were systematically investigated.

The detection limit and response rate of BP

sensors were superior to those of graphene based sensors.

Heavy metal ions can be

effectively detected over a wide concentration range, and the relative conduction change of BP followed the Langmuir isotherm.

The results presented here reveal important sensing

characteristics of intrinsic black phosphorus, which can motivate further fundamental studies and potential applications.

ASSOCIATED CONTENT Supporting information Black phosphorus synthesis, ionophore membrane composition, I-V characters of BP FET, failure of BP sensor, black phosphorus degradation and ionophore protection, detection of K+ and Na+ ions. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]



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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The work described in this paper was supported in part by the National Natural Science Foundation of China (No. 51405257, 51407200) and Science Fund of China Post Doctor (No. 2012M520259).

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List of Figures Figure 1. (a) BP sensor fabrication process.

Multilayer BP flakes were mechanically

exfoliated by scotch tape based method and transferred onto SiO2/Si substrate, followed by metal electrode fabrication process. BP surface.

Sequentially, ionophore solution was spun coated on

(b) AFM topographic image (9×9 µm2) of a multilayer BP flake.

height profile of the BP flake extracted from b, demonstrating a height of 12 nm.

(c) AFM (d) Raman

spectrum of a multilayer BP flake which shows three characteristic peaks (A1g at ~362 cm-1, B2g at ~440 cm-1, and A2g at ~466 cm-1).

(e) Schematic view of a BP sensor.

The BP flake

is covered with ionophore film. Figure2. (a)-(d) Optical images of an unprotected BP flake on day 1, 4, 15, and 30 respectively. as well.

Degradation bubbles were clearly observed, and the color of the flake changed

(e), (f) Optical images of a BP capped with ionophore on day 1 and day 30,

respectively, demonstrating no obvious optical evidence for degradation.

(g) IDS vs VGS

curves of BP without ionophore protection on day 1, 2 and 3, respectively, demonstrating a clear drop of source-drain current IDS.

(h) IDS vs VGS curves of BP with ionophore protection.

IDS variation is less than 10%, suggesting significantly improved air stability. Figure 3. (a) IDS vs VGS curves of BP with different Pb2+ concentrations. vs VGS curves of BP with different Pb2+ concentrations. BP with different Pb2+ concentrations.

VDS=0.5 V.

VDS=1 V.

VDS=3 V.

(b) IDS

(c) IDS vs VGS curves of

(d) IDS vs VGS curves of BP with

different Pb2+ concentrations.

VDS=-2.4 V.

concentrations.

(f) IDS vs VGS curves of BP with different Pb2+ concentrations.

VDS=-3.2 V.

(e) IDS vs VGS curves of BP with different Pb2+

VDS=-4 V. Figure 4. (a) The output signal ∆IDS of BP sensor for 1 ppb Pb2+ is 3.3 nA, and the standard deviation of IDS noise is 0.34 nA. ppb.

The detection limit of BP sensor for Pb2+ ion can reach 1

(b) Relative resistance change (∆R/R0) of BP versus time (VGS =0 V).

in the inset demonstrates the time constant is 5 s. (experimental results from 7 sensors). concentrations.

The curve fit

(c) ∆R/R0 versus Pb2+ concentration

Resistance of BP shows saturated at higher

Experimental results demonstrate good linear relation between


∆

and C,


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fitting the Langmuir adsorption isotherm well, as shown in the inset. Figure 5. (a) Schematic view of BP sensors with lead ionophore and arsenite ionophore, respectively.

Theoretically, ionophore only allows one type of molecules to pass through it,

and blocks all the other ions and molecules.

The Pb2+ ions serve as a positive gate voltage

on BP surface, which repels positively charged holes, while AsO2- attracts more holes in BP. (b) Band structure of BP.

After Pb2+ absorption, Fermi energy moves upward, reducing the

hole density and the conductivity.

After AsO2- absorption, Fermi energy moves downward,

increasing the hole density and the conductivity of BP. Figure 6. Different heavy metal ions detected by BP sensors (VDS=-3 V).

(a) Different

concentrations of AsO2- detected by BP sensors capped with arsenite ionophore. absorption of AsO2- increased IDS.

The

(b) Different concentrations of Cd2+ detected by BP

sensors capped with cadmium ionophore.

The absorption of Cd2+ decreased IDS.

(c)

Different concentrations of Hg2+ detected by BP sensors capped with mercury ionophore. (d) AsO2- detected by BP sensors with lead ionophore. lead ionophore.

(e) Cd2+ detected by BP sensors with

(f) Hg2+ detected by BP sensors with lead ionophore.


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Figure 1. (a) BP sensor fabrication process.

Multilayer BP flakes were mechanically

exfoliated by scotch tape based method and transferred onto SiO2/Si substrate, followed by metal electrode fabrication process. BP surface.

Sequentially, ionophore solution was spun coated on

(b) AFM topographic image (9×9 µm2) of a multilayer BP flake.

height profile of the BP flake extracted from b, demonstrating a height of 12 nm.

(c) AFM (d) Raman

spectrum of a multilayer BP flake which shows three characteristic peaks (A1g at ~362 cm-1, B2g at ~440 cm-1, and A2g at ~466 cm-1).

(e) Schematic view of a BP sensor.

is covered with ionophore film.


The BP flake


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Figure2. (a)-(d) Optical images of an unprotected BP flake on day 1, 4, 15, and 30 respectively. as well.

Degradation bubbles were clearly observed, and the color of the flake changed

(e), (f) Optical images of a BP capped with ionophore on day 1 and day 30,

respectively, demonstrating no obvious optical evidence for degradation.

(g) IDS vs VGS

curves of BP without ionophore protection on day 1, 2 and 3, respectively, demonstrating a clear drop of source-drain current IDS.

(h) IDS vs VGS curves of BP with ionophore protection.

IDS variation is less than 10%, suggesting significantly improved air stability.


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Figure 3. (a) IDS vs VGS curves of BP with different Pb2+ concentrations. vs VGS curves of BP with different Pb2+ concentrations. BP with different Pb2+ concentrations.

VDS=0.5 V.

VDS=1 V.

VDS=3 V.

(b) IDS

(c) IDS vs VGS curves of

(d) IDS vs VGS curves of BP with

different Pb2+ concentrations.

VDS=-2.4 V.

concentrations.

(f) IDS vs VGS curves of BP with different Pb2+ concentrations.

VDS=-3.2 V.

(e) IDS vs VGS curves of BP with different Pb2+

VDS=-4 V.



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Figure 4. (a) The output signal ∆IDS of BP sensor for 1 ppb Pb2+ is 3.3 nA, and the standard deviation of IDS noise is 0.34 nA. ppb.

The detection limit of BP sensor for Pb2+ ion can reach 1

(b) Relative resistance change (∆R/R0) of BP versus time (VGS =0 V).

in the inset demonstrates the time constant is 5 s. (experimental results from 7 sensors). concentrations.

The curve fit

(c) ∆R/R0 versus Pb2+ concentration

Resistance of BP shows saturated at higher

Experimental results demonstrate good linear relation between

fitting the Langmuir adsorption isotherm well, as shown in the inset.


∆

and C,

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Figure 5. (a) Schematic view of BP sensors with lead ionophore and arsenite ionophore, respectively.

Theoretically, ionophore only allows one type of molecules to pass through it,

and blocks all the other ions and molecules.

The Pb2+ ions serve as a positive gate voltage

on BP surface, which repels positively charged holes, while AsO2- attracts more holes in BP. (b) Band structure of BP.

After Pb2+ absorption, Fermi energy moves upward, reducing the

hole density and the conductivity.

After AsO2- absorption, Fermi energy moves downward,

increasing the hole density and the conductivity of BP.



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Figure 6. Different heavy metal ions detected by BP sensors (VDS=-3 V).

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(a) Different

concentrations of AsO2- detected by BP sensors capped with arsenite ionophore. absorption of AsO2- increased IDS.

The

(b) Different concentrations of Cd2+ detected by BP

sensors capped with cadmium ionophore.

The absorption of Cd2+ decreased IDS.

(c)

Different concentrations of Hg2+ detected by BP sensors capped with mercury ionophore. (d) AsO2- detected by BP sensors with lead ionophore. lead ionophore.

(e) Cd2+ detected by BP sensors with

(f) Hg2+ detected by BP sensors with lead ionophore.


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69x29mm (600 x 600 DPI)



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Figure 1. (a) BP sensor fabrication process. Multilayer BP flakes were mechanically exfoliated by scotch tape based method and transferred onto SiO2/Si substrate, followed by metal electrode fabrication process. Sequentially, ionophore solution was spun coated on BP surface. (b) AFM topographic image (9×9 µm2) of a multilayer BP flake. (c) AFM height profile of the BP flake extracted from b, demonstrating a height of 12 nm. (d) Raman spectrum of a multilayer BP flake which shows three characteristic peaks (A1g at ~362 cm-1, B2g at ~440 cm-1, and A2g at ~466 cm-1). (e) Schematic view of a BP sensor. The BP flake is covered with ionophore film. 118x141mm (600 x 600 DPI)


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Figure2. (a)-(d) Optical images of an unprotected BP flake on day 1, 4, 15, and 30 respectively. Degradation bubbles were clearly observed, and the color of the flake changed as well. (e), (f) Optical images of a BP capped with ionophore on day 1 and day 30, respectively, demonstrating no obvious optical evidence for degradation. (g) IDS vs VGS curves of BP without ionophore protection on day 1, 2 and 3, respectively, demonstrating a clear drop of source-drain current IDS. (h) IDS vs VGS curves of BP with ionophore protection. IDS variation is less than 10%, suggesting significantly improved air stability. 67x45mm (600 x 600 DPI)



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Figure 3. (a) IDS vs VGS curves of BP with different Pb2+ concentrations. VDS=3 V. (b) IDS vs VGS curves of BP with different Pb2+ concentrations. VDS=1 V. (c) IDS vs VGS curves of BP with different Pb2+ concentrations. VDS=0.5 V. (d) IDS vs VGS curves of BP with different Pb2+ concentrations. VDS=-2.4 V. (e) IDS vs VGS curves of BP with different Pb2+ concentrations. VDS=-3.2 V. (f) IDS vs VGS curves of BP with different Pb2+ concentrations. VDS=-4 V. 55x30mm (600 x 600 DPI)


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Figure 4. (a) The output signal ∆IDS of BP sensor for 1 ppb Pb2+ is 3.3 nA, and the standard deviation of IDS noise is 0.34 nA. The detection limit of BP sensor for Pb2+ ion can reach 1 ppb. (b) Relative resistance change (∆R/R0) of BP versus time (VGS =0 V). The curve fit in the inset demonstrates the time constant τ is 5 s. (c) ∆R/R0 versus Pb2+ concentration (experimental results from 7 sensors). Resistance of BP shows saturated at higher concentrations. Experimental results demonstrate good linear relation between C/( ∆G) and C, fitting the Langmuir adsorption isotherm well, as shown in the inset. 48x14mm (600 x 600 DPI)



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Figure 5. (a) Schematic view of BP sensors with lead ionophore and arsenite ionophore, respectively. Theoretically, ionophore only allows one type of molecules to pass through it, and blocks all the other ions and molecules. The Pb2+ ions serve as a positive gate voltage on BP surface, which repels positively charged holes, while AsO2- attracts more holes in BP. (b) Band structure of BP. After Pb2+ absorption, Fermi energy moves upward, reducing the hole density and the conductivity. After AsO2absorption, Fermi energy moves downward, increasing the hole density and the conductivity of BP. 57x33mm (600 x 600 DPI)


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Figure 6. Different heavy metal ions detected by BP sensors (VDS=-3 V). (a) Different concentrations of AsO2- detected by BP sensors capped with arsenite ionophore. The absorption of AsO2- increased IDS. (b) Different concentrations of Cd2+ detected by BP sensors capped with cadmium ionophore. The absorption of Cd2+ decreased IDS. (c) Different concentrations of Hg2+ detected by BP sensors capped with mercury ionophore. (d) AsO2- detected by BP sensors with lead ionophore. (e) Cd2+ detected by BP sensors with lead ionophore. (f) Hg2+ detected by BP sensors with lead ionophore. 88x48mm (600 x 600 DPI)


Air-Stable Black Phosphorus Devices for Ion Sensing - ACS Applied

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