As it is known, the work function is the minimum quantity of energy needed to move an electron from the surface of a conductor material up to an external point close to the metal with a kinetic energy equal to zero. In particular, the work function is defined as the difference between the Fermi level and the void level, in other words, as the energy level associated with the external space around the metal.

Since the energy levels of the electrons are affected by optic, electrical and mechanical characteristics of the material, the work function appears to be an extremely sensitive indicator of the material characteristics and in particular of its surface.

The technique of measuring work function of unknown materials, conceived in the nineteenth century by the Scottish scientist Lord Kelvin, also known as Kelvin probe technique, is still one of the today's most used techniques, because of its conceptual and technological simplicity and also because of its low implementation costs.

In particular, the Kelvin probe is substantially a non-destructive vibrant capacitor device not in contact with the surface of the material of which the work function is to be measured, which allows measuring the difference of the work function of metals (or the surface potential for non- metals) between a plate made of the conductor material under measurement and a vibrant armor of reference.

Figure 1 shows a circuit schematic model of a measure carried out with a conventional Kelvin probe, wherein it may be observed a series circuit formed by a generator 7 of adjustable supply voltage Vapp, a capacitor 1, a Kelvin virtual generator 2 (that schematises the difference of the work functions between the capacitor plates) and an electronic circuit 3 able to process data and to output a voltage Vout the value of which is dependent on the work function of the material under measurement.

As said before, the capacitor 1 includes a reference plate 4, preferably made of gold, that is forced to vibrate at a determined

frequency, preferably by a piezoelectric system, and a plate 5 made of the material of which the work function is to be determined. Because of the vibration of the reference plate 4, the distance d between the two plates 4 and 5 of the capacitor 1 periodically changes around a mean distance do with an oscillating amplitude dM.

The electronic circuit 3 is represented in Figure 1 by an operational amplifier 6 in an inverting configuration, the output voltage Vout of which is a function of the input current l (t), which is in turn dependent on the difference between the work functions of the materials of the two plates 4 and 5 of the capacitor 1 Since the work function of the reference material, preferably gold, is known with good precision, the output voltage Vout obtained through the Kelvin measure gives, by difference, a measure of the work function of the unknown material of the plate 5.

In fact, the current l (t) to be detected is equal to the temporal variation of the electric charge Q on the plates 4 and 5 of the capacitor 1.

Since, as known, the electric charge Q is in turn equal to the product of the capacitance C of the capacitor 1 by the voltage Vc applied to the latter, the current I (t) is expressed by the following formula: I (t) = dQ/dt = C dVc/dt + Vc dC/dt where the voltage Vc applied to the capacitor 1 is equal to the difference between the voltage Vapp and the difference A (j) (schematised in Figure 1 by the virtual generator 2) between the work functions of the materials of the two plates 4 and 5 of the capacitor 1: Vc =Vapp-Af In the Kelvin probe technique, the adjustable supply voltage Vapp of the generator 7 is caused to change from a negative value to a positive one and, in the proximity of the zero value of the current l (t), it is carried out the reading of the applied supply voltage Vapp, also called as Kelvin voltage, from which it is possible to obtain the work function of the unknown material of the plate 5.

Nowadays available equipments implementing the Kelvin probe technique suffer from some drawbacks.

First of all, these equipments operate making the capacitor 1 vibrate at a fixed frequency, equal to about 180'Hz, which represents the mechanical resonating frequency of the capacitor 1.

Furthermore, in the case of application of the technique to

liquids the work function of which is to be measured, the movement of the reference plate 4 of the capacitor 1 can affect the real value of the work function of the liquid surface under measurement.

It is therefore an object of the present invention to provide an apparatus for measuring work functions of materials which allows, in a precise, simple, efficient, reliable and inexpensive way, to obtain the work function value of conductor or semi conductor materials under measurement, also including liquid surfaces of any nature, in particular biological liquids.

It is still an object of the present invention to provide such an apparatus that has high sensitivity.

It is specific subject matter of this invention an apparatus for measuring work functions of materials, comprising at least one reference plate and at least one housing, apt to house a material, the work function of which is to be measured, so that the material housed in said at least one housing constitutes at least one second plate facing said at least one reference plate so forming at least one measuring capacitor, the apparatus being characterised in that said at least one reference plate is at a fixed distance from said at least one housing, and in that it comprises at least one further capacitor having variable capacitance, that is connected in series to said at least one reference plate and to said at least one housing.

Always according to the invention, said at least one further capacitor may include two plates at least one of which is mobile with respect to the other. In particular, said at least one of said two plates may be mobile with respect to the other in any direction.

Still according to the invention, each one of said two plates may be made of a metallic material.

Furthermore according to the invention, said metallic material may be gold or platinum.

Always according to the invention, said two plates may be made of the same material.

Still according to the invention, said at least one reference plate may be made of the same material of which said two plates are made.

Furthermore according to the invention, each one of said two plates may be made of a material different with respect to the other one.

Always according to the invention, said at least one further capacitor may include a mobile dielectric layer.

Still according to the invention, the apparatus according to the invention may further comprise means for controlling the capacitance of said at least one further capacitor.

Furthermore according to the invention, said controlling means may periodically change the capacitance of said at least one further capacitor.

Always according to the invention, said controlling means may comprise piezoelectric means.

Still according to the invention, said fixed distance, at which said at least one reference plate and said at least one housing are, may be adjustable.

Furthermore according to the invention, at least one housing may be apt to house a liquid material.

Always according to the invention, said liquid material may be a biological liquid material.

Still according to the invention, the apparatus according to the invention may further comprise supply voltage generating means, connected in series to said at least one further capacitor, to said at least one reference plate, and to said at least one housing.

Furthermore according to the invention, said supply voltage generating means may be apt to generate an adjustable voltage.

Always according to the invention, said supply voltage generating means may be apt to generate a voltage that is constant in time.

Still according to the invention, said supply voltage generating means may be apt to generate a voltage that is variable in time.

Furthermore according to the invention, the apparatus according to the invention may further comprise processing electronic means apt to provide, when said at least one housing houses a material, an output signal the value of which is indicative of the work function of the material housed in said at least one housing.

Always according to the invention, said output signal may be a function of the value of the total capacitance of the series connection of said at least one measuring capacitor, formed by the material housed in said at least one housing and said at least one reference plate, with said at least one further capacitor.

In particular, said at least one reference plate may be either a

tip or a slab, preferably metallic.

Also, in the case when said at least one reference plate is a tip, the apparatus may further comprise means for moving it and said processing electronic means may control said means for moving said at least one reference plate so as to make it scan all the surface of the material housed in said at least one housing. In particular, said at least one reference plate can be moved by a piezoelectric system in order to locate the correct position on the surface under test.

Always according to the invention, the area of said at least one reference plate can be as small as 0.1 microns sq.

Still according to the invention, said processing electronic means may comprise current measuring means.

The present invention will now be described, by way of illustration and not by way of limitation, according to its preferred embodiment, by particularly referring to the Figures of the enclosed drawings, in which: Figure 1 shows a schematic circuit model of a work function measurement carried out with a conventional Kelvin probe; and Figure 2 shows a schematic circuit model of a work function measurement carried out with a preferred embodiment of the apparatus according to the invention.

In the Figures, alike elements are indicated by the same reference numbers.

Referring to Figure 2, it may be observed a schematic circuit model of a work function measurement carried out with a preferred embodiment of the apparatus according to the invention, which includes a first capacitor 10, a reference plate 14, and a housing (not shown in the Figure) made of a conductor material apt to house a material the work function of which is to be determined in such a way that the latter is a second plate 15 facing the reference plate 14, so forming a second capacitor 11.

The reference plate 14 and said housing are connected to the first capacitor 10 in such a way that the second capacitor 11 is connected in series to the first capacitor 10.

The first capacitor 10 includes two plates 12 and 13 made of the same material, preferably gold or platinum, one of which (the plate indicated in Figure 2 with reference number 12) is mobile with respect to

the other, being forced to vibrate at a determined frequency, preferably through a piezoelectric system.

The reference plate 14 is made of a material having known work function, preferably gold or platinum The apparatus of Figure 2, that also schematises a Kelvin virtual generator 2 modelling the difference between the work functions of the plates 14 and 15 of the second capacitor 11, further includes a generator 7 of adjustable supply voltage Vapp, and an electronic circuit 3 capable to process data and to output a voltage Vout the value of which is function of the work function of the material under measurement of the plate 15. All these components, similar to those already illustrated with reference to Figure 1, are connected in series to the first capacitor 10 and the second capacitor 11.

It is evident to those skilled in the art that the electronic circuit 3, represented in Figure 2 by an operational amplifier 6 in an inverting configuration, can be also implemented in a different way. In particular, the current gauge 2 and the electronic circuit 3 can be integrated in a however complex single processing unit.

In detail, the apparatus according to the invention detects the current l (t) flowing along the circuit loop represented in Figure 2. It is equal to the time variation of the electric charge Q on the plates of the capacitors 10 and 11, that in turn is equal to the product of the total capacitance Ctot of the series connection of the first capacitor 10 with the second capacitor 11, by the voltage Vctot applied to such series connection: I (t) = dQ/dt = Ctot dVctot/dt + Vctot dCtot/dt In particular, the total capacitance Ctot of the series connection between the two capacitors 10 and 11 is equal to: Ctot = CiC2/ (Ci+C2), where C1 is the value, variable in time, of the capacitance of the first capacitor 10, and C2 is the value, constant in time, of the capacitance of the second capacitor 11.

The voltage Vctot applied to the series connection between the two capacitors 10 and 11 is equal to the difference between the supply voltage Vapp and the difference A (j) of the work functions of the materials of the two plates 14 and 15 of the second capacitor 11 : Vctot = Vapp-tf Other embodiments of the present invention may provide that

the first capacitor 10 includes two plates of different materials having known work functions. In this case, the voltage Vctot applied to the series connection is obtained by also subtracting the value of the difference of the known work functions of the two materials of the plates of the first capacitor 10 from the voltage Vapp.

The advantages offered by the apparatus according to the invention are evident.

In fact, the second capacitor 11, having a plate made of the material under measurement housed in said housing, has no mobile plates and, therefore, it allows in a reliable and precise way to also determine the work functions of liquid surfaces of any nature, biological liquids included.

Moreover, the first capacitor 10 having at least one mobile plate with respect to the other can be made in such a way that its mechanical resonance frequency may substantially assume any desired value.

Consequently, the apparatus according to the invention is highly flexible.

The preferred embodiments have been above described and some modifications of this invention have been suggested, but it should be understood that those skilled in the art can make other variations and changes, without so departing from the related scope of protection, as defined by the following claims.