Juan Manuel Conde Garrido*, Josefina María Silveyra, María Andrea Ureña, Bibiana Arcondo

INTECIN (UBA-CONICET), Faculty of Engineering, University of Buenos Aires, Argentina * jmcondegarrido@fi.uba.ar

In the past three decades, solid-state Ion Selective Electrodes (ISEs) have incorporated advances in nanotechnology by using chalcogenide glasses as sensitive membranes. The chemical stability chalcogenide glasses guarantees high durability even in acid environments. These materials are easily synthetized and can therefore be used in the form of both bulk and thin films for developing sensitive membranes for low-dimensional sensors and multisensors arrangements.

In this work, chalcogenide glasses of composition Agx(Ge0.25Se0.75)100-x (0 at. % ≤ x ≤ 25 at. %) were studied as a sensitive material for ISEs. • For x ≥ 8 at. % the glasses are sensitive to Ag+ and Cu2+ ions in water-based solutions. • For x < 8 at. % the electrodes response is poor.

In order to understand the sensing properties of chalcogenide glasses, it is first necessary to know their electric properties. The goal of this work was to clarify if the sensing property of these materials in exclusively determined by the ionic conductivity or by the total resistance of the

material. The materials were prepared in the form of bulk and thin films membranes. Thin films of some hundreds of nanometers thick were synthetized by Puled Laser

Deposition (PLD). • For x ≥ 8 at. %, the glasses system behaved as a ionic conductor (10-5 S/cm) • For x < 8 at. % the system behaved as an insulator (<10-13 S/cm)

Synthesis conditions: Vacuum: ~10-6 mbar Pure elements (4N) Liquid temperature: 970 °C Time in the liquid state: 8 h

AgX(Ge0.25Se0.75)100-x (0 at. % ≤ x ≤ 25 at. %)

0 20000 40000 60000 80000 100000

0

10000

20000

30000

40000

50000

60000

Ag 15

T = 21OC T = 25OC T = 30OC T = 40OC T = 50OC T = 60OC T = 70OC T = 80OC T = 90OC

- Z´

( Ω )

Z ( Ω )

ω

The jump on conductivity with Ag content variation observed in the right figure above, can be explained by the heterogeneous character of the amorphous sample: The glass consists in a minority amorphous rich in Ag phases embedded in non conductive amorphous matrix. The characteristic dimension of the Ag-rich amorphous phase is between the micro- and the nanometer scale and percolates when the Ag content of the material is ~ 7.5 %at. This percolation allows the conduction of Ag+ ions through the material.

Conductivity measurements of bulk samples by Impedance Spectroscopy (IS)

Tamb

We thank Dra. Liliana Fraigi and Ms. Eliana Mangano (INTI) for the deposition of Cr contacts.

The acquiring software was developed in LabView platform. The program controls an electrometer (Keithley 6517A) and acquires data. A constant voltage is applied and the current is measured. Then, voltage polarity is reversed and the current is measured again.

Glass samples were obtained by melt quenching

from the liquid state.

Data obtained by the Isothermal Transient Ionic Current (ITIC) technique: Applied voltage = 1 V tpeak = 6.309 s Thickness = 1.549 mm

PLD Thin films deposited by PLD with Cr electrodes deposited by sputtering

The ionic nature of the majority of charge carriers in both bulk and thin films was shown.

Deposition conditions: Vacuum: ~10-6 mbar Substrate-Target distance: 4 cm Deposition time: 30 min

Pulsed laser Nd:YAG (λ: 355 nm) Pulse duration: 5 nsec Repetition: 10 Hz

ITIC technique allows to determine ionic mobility (µ) by the time in which the maximum current takes place since the polarity reversal (τ):

)(

2

efVVd−

=τ

µ

V: applied voltage Vef: voltage decrease due to the charge spatial distribution effect d: distance between the measurement contacts

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

2

4

6

8

10

12

Equation y = a + bAdj. R-Squa 0.88189

Value Standard Errd2/tau/e-9 Intercept -2.7418 1.08041d2/tau/e-9 Slope 18.8712 2.06969

d2 /τ*10

9 cm

2 s-1

Voltage (V)

Vef=0,145 V

µ = 1,88.10-8cm2V-1s-1

0 5000 10000 15000 20000

-0.000006

-0.000004

-0.000002

0.000000

0.000002

0.000004

0.000006

Curre

nt (A

)

Time (s)

Ag 9 (bulk)

)(

2

efVVd−

=τ

µ

Ag10 thin film

The ion mobility (µ) was estimated through the following equation: where σ is the conductivity obtained from IS measurements, Ze is the ion electric charge and N is the mobile ion concentration

µσ ZeN=

Ag10 Ag15 Ag20 Ag25 µ

cm2V-1s-1 1.6 10-8 4.4 10-8 5.1 10-8 7.1 10-8

The mobility estimated for the Ag10 thin film is close to the mobility estimated for the same material in bulk.

Data obtained by the Isothermal Transient Ionic Current (ITIC) technique: Applied voltage = 0.5 V tpeak = 0,401 s Thickness (d) ≈ 500 nm