SYSTEM FOR SCANNING PROBE MICROSCOPY APPLICATIONS AND METHOD FOR OBTAINING SAID SYSTEM

Abstract

The invention relates to a system suitable for its use in scanning probe microscopy, such as tip-enhanced Raman spectroscopy or magnetic force microscopy, that comprises: a tip (1) comprising an apex (1′); a plurality of nanoparticles (2, 2′) attached to the tip (1); having a size between 0.5 and 100 nm. Advantageously, the plurality of nanoparticles (2, 2′) comprises a cluster (2″) of one or more nanoparticles (2′) disposed at the apex (1′) of the tip (1), wherein the cluster (2″) is spaced from any other nanoparticle (2) of the tip (1) at least a distance d of 0.5 nm. The invention also relates to a method for obtaining such system through a controlled thermal treatment that exploits the intrinsic properties of nanoparticles.

Claims

1. A method of production of a system suitable for its use in scanning probe microscopy, such as tip-enhanced Raman spectroscopy or magnetic force microscopy, said method being characterized in that it comprises the following steps: a) providing a tip comprising an apex; b) depositing a plurality of nanoparticles with a size of between 0.5 and 100 nm on the tip; c) applying a thermal treatment on the tip and nanoparticles deposited on the tip in the previous step b) and reaching a melting temperature of one or more nanoparticles, said temperature being between 320 and 1275 K, and maintaining such temperature, so that the plurality of nanoparticles suffers a change in its diameter distribution and nanoparticle density forming a cluster, wherein said cluster is spaced from any other nanoparticle of the tip at least a distance d of 0.5 nm, and wherein a mean separation between nearest neighboring nanoparticles in said cluster is less than 0.5 nm; d) cooling the tip and nanoparticles down to room temperature.

2. The method according to claim 1, wherein the thermal treatment of step c) is applied until the cluster is spaced from any other nanoparticle of the tip the distance d of at least 1 nm, 5 nm, 10 nm, 100 nm or 1000 nm.

3. The method according to claim 1, wherein in step b), the procedure for the deposition of the plurality of nanoparticles is one or a combination of the following procedures: sol-gel deposition, deposition of nanoparticles from a solution, gas-phase deposition procedures, or any procedure comprising the deposition of nanometric clusters with a nanoparticle size between 0.5 and 100 nm, under atmospheric pressure, in vacuum, in high-vacuum or ultra-high-vacuum.

4. The method according to claim 1, wherein in step c), the thermal treatment comprises one or more of the following treatments: electron beam or photon beam treatments, laser or microwave treatments, treatments by using lamps emitting in a selected wavelength range, furnaces or heating plates.

5. The method according to claim 1, wherein the thermal treatment of step c) lasts a period of time between 1 ms and 2 hours.

6. The method according to claim 1, wherein the cooling step d) lasts a period of time between 10 seconds and 2 hours.

7. A system suitable for its use in scanning probe microscopy, such as tip-enhanced Raman spectroscopy or magnetic force microscopy, directly obtained through a method according to any of the preceding claims, comprising: a tip comprising an apex; a plurality of nanoparticles attached to the tip; having a size between 0.5 and 100 nm; said system being characterized in that: the plurality of nanoparticles comprises a cluster of two or more nanoparticles disposed at the apex of the tip, wherein said cluster is spaced from any other nanoparticle of the tip at least a distance d of 0.5 nm, and a mean separation between nearest neighboring nanoparticles in said cluster is less than 0.5 nm.

8. The system according to claim 7, wherein the cluster is spaced from any other nanoparticle of the tip a distance d of at least 1 nm, 5 nm, 10 nm, 100 nm or 1000 nm.

9. The system according to claim 7, wherein the nanoparticles comprise an electrically conductive material.

10. The system according to claim 9, wherein the nanoparticles comprise Au, Ag or a combination of Au and Ag.

11. The system according to claim 7, wherein the nanoparticles comprise a ferromagnetic, antiferromagnetic and/or superparamagnetic material.

12. The system according to claim 12, wherein the nanoparticles comprise Co, Fe or a homogeneous or heterogeneous alloy comprising Co and/or Fe.

13. The system according to claim 7, wherein the nanoparticles have a core-shell structure and/or a Janus structure.

14. The use of a system according to claim 7 for any of the following techniques: magnetic force microscopy, tip-enhanced Raman spectroscopy, nano infrared microscopy, Kelvin probe force microscopy, piezoresponse force microscopy or scanning capacitance microscopy.

Description

DESCRIPTION OF THE DRAWINGS

[0045] The characteristics and advantages of this invention will be more apparent from the following detailed description, when read in conjunction with the accompanying drawings, in which:

[0046] FIG. 1 shows the evolution of the melting temperature of gold nanoparticles vs. nanoparticle diameter (source: Physical Review A 13, 2287 (1976)).

[0047] FIG. 2 shows a diagram of an SPM tip (FIG. 2a), a detail of the cantilever and tip apex with nanoparticles before the thermal treatment of the method of the invention (FIG. 2b) and after (FIG. 2c), showing the melting of nanoparticles (NPs) until a cluster comprising only one nanoparticle is disposed in the tip apex, isolated from other nanoparticles (relative to the near-field), suitable for acting as an enhanced probe in SPM.

[0048] FIG. 3 shows a cantilever for SFM microscopy applications, comprising a system according to the invention. The system comprises a tip with a thermal gradient (the darker the nanoparticle, the higher the reached temperature) due to the geometry of the tip and the applied thermal treatment of the method of the invention.

[0049] FIGS. 4a and 4b show two schematic different systems obtained after the applied thermal treatment: (a) a system with only one nanoparticle in the cluster isolated at the tip apex and (b) another system with a group of nanoparticles in the cluster isolated at the tip apex, according to the invention. Spacing distance d is shown in both cases.

NUMERICAL REFERENCES USED IN THE DRAWINGS

[0050] In order to provide a better understanding of the technical features of the invention, the referred FIGS. 1-4 are accompanied of a series of numeral references which, with illustrative and non limiting character, are hereby represented:

TABLE-US-00001 (1) Tip (1’) Tip apex (2) Nanoparticles (2’) Nanoparticles disposed in the tip apex (2”) Cluster of one or more nanoparticles (3) Cantilever

DETAILED DESCRIPTION OF THE INVENTION

[0051] In the following description, for purposes of explanation and not limitation, details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these details and descriptions without departing from the spirit and scope of the invention. Certain embodiments will be described below with reference to the drawings (FIG. 1-4) wherein illustrative features are denoted by reference numerals.

[0052] As described in previous sections, a main object of the invention is related to a method for the fabrication and modification of Scanning Probe Microscopy (SPM) probes through the coating of nanoparticles.

[0053] In a preferred embodiment of the invention, the AFM tip (1) is a tip made of silicon, silicon nitride or silicon oxide, and comprises an apex (1′) (see FIGS. 2-4). Also, in a further embodiment of the invention, the tip is not subject to any physical or chemical pre-treatment before the deposition of nanoparticles (2).

[0054] The nanoparticles (2) for the coating can be made of one and/or more elements, which can be: electrically conductive, semiconductors, insulators, ferromagnetic, antiferromagnetic, superparamagnetic, ferrimagnetic, paramagnetic, magneto-optic, piezoelectric, fluorescent, superconductors or any combination thereof.

[0055] In a preferred embodiment of the invention, the materials used for coating AFM tips (1) with nanoparticles (2) are gold (Au), silver (Ag) or a combination of them, mainly because the excitation of the plasmon resonances occurring in the nanoparticles (2) of such materials. Preferably, gold containing nanoparticles are chosen for the coating because their better oxidation resistance in comparison to pure silver, which oxides easily during measurement process and can be used only for a few hours.

[0056] In a preferred embodiment of the invention, the materials used for coating AFM tips (1) with nanoparticles (2) are cobalt (Co), iron (Fe) or alloys comprising Co or Fe or core-shell structures comprising Co or Fe.

[0057] In a preferred embodiment of the invention, the nanoparticles are made of a combination of the previous embodiments (electrically conductive material and magnetic material).

[0058] As an example, any material being electrically conductive and/or presenting a surface plasmon resonance is interesting for its use in applications combining AFM with spectroscopy as TERS or nano-infrared (nanoIR) for sample characterization. On the other hand, any material or combination of materials with magnetic properties as Co, Fe, or any of their alloys are interesting for applications which combine AFM with other interaction forces as occur in MFM.

[0059] In a preferred embodiment of the invention, the nanoparticles (2) have a Janus structure, where their surfaces have two or more distinct physical properties. This structure allows two or more different types of chemistry to occur on each same nanoparticle (2).

[0060] In a preferred embodiment of the invention, the nanoparticles (2) are made of a homogeneous or heterogeneous alloy.

[0061] In a preferred embodiment of the invention, the metallic nanoparticles (2) deposition can be made in a deposition chamber containing an ion cluster source (ICS). This method allows the homogeneous and random deposition of nanoparticles (2) over the AFM tip (1) surface. Alternatively, other deposition methods can be employed, such as sol-gel deposition, deposition of nanoparticles (2) in a solution, gas-phase deposition procedures, or any procedure comprising the deposition of nanometric clusters or a nanoparticle (2) with size between 0.5 and 100 nm, under atmospheric pressure, in high pressure, in vacuum, in high-vacuum or ultra-high-vacuum.

[0062] In a preferred embodiment, the deposited nanoparticles (2) have an original size equal or less than 100 nm. Typically, the nanoparticles (2) have spherical form and/or a diameter of 4-6 nm.

[0063] The main idea underneath the present method of the invention is the exploitation of the intrinsic melting properties of very small nanoparticles (2) (size under 100 nm) compared to their bulk form, as shown in FIG. 1, where it is presented the evolution of the melting temperature of a nanoparticle (2) vs. the size or diameter of a nanoparticle (2). As shown, the smaller the nanoparticle (2), the lower the melting temperature, especially with a nanoparticle (2) size under 20 nm. This is, depending on the size of the nanoparticle (2) and the density of nanoparticles per surface area, at a defined temperature, nanoparticles (2) tend to coalesce nearby nanoparticles (2) (see FIGS. 2a-2c) increasing their dimension to maintain the equilibrium in the thermodynamic system. In this way, a thermal treatment applied to the nanoparticles (2) involves an increase in the temperature which causes an increase in the nanoparticle size and, consequently, a lower density of nanoparticles (2).

[0064] In different embodiments of the invention, the thermal treatment comprises one or more of the following treatments: electron beam or photon beam treatments, laser or microwave treatments, treatments by using lamps emitting in a selected wavelength range, furnaces, heating plates or any other thermal treatment capable of heating the nanoparticles (2) at least until their melting temperature.

[0065] Regarding the aforementioned intrinsic properties of nanoparticles (2) and as a fundamental summary of information in order to understand the method of the present invention, it can be stated that: [0066] A surface coated with nanoparticles (2) can be modified with thermal treatments that change the final characteristics of the nanoparticles (2). [0067] If the applied temperature to a surface coated with nanoparticles (2) is above the melting temperature of the nanoparticles (2), those nanoparticles (2) melt forming larger nanoparticles (2) if they are close enough. [0068] After the melting, the larger nanoparticles (2) present a higher melting temperature (FIG. 1). As a consequence, if the applied temperature does not surpass the new melting temperature, the fusion process stops, resulting in a surface coated with nanoparticles (2) larger than the original ones. [0069] Thus, the final size of the nanoparticles (2) can be controlled through the applied temperature. [0070] Such thermal treatment can be applied through several techniques: electron beam or photon beam treatments, laser or microwave treatments, treatments by using lamps emitting in a selected wavelength range, furnaces, heating plates or any other thermal procedure. [0071] Such thermal treatment can be applied to nanoparticles (2) of any chemical composition and also to nanoparticles (2) deposited on any surface, by any deposition method. [0072] The morphology of the surface coated with nanoparticles also influences the melting point of the nanoparticles. In a morphology like a tip (1), the temperature reached at the tip apex (1′) is higher than in the basement of the tip (1), as depicted in FIG. 3, thus, reaching the melting and coalescence of the nanoparticles at the apex (1′) before other parts of the tip (1) or the flat surface of the cantilever.

[0073] In a preferred embodiment of the invention, the method makes the most of the afore described intrinsic properties of nanoparticles in order to modify AFM tips (1) with nanoparticles (2) by means of a thermal treatment on the tip (1) and the nanoparticles (2). Preferably, the thermal treatment allows locating an isolated single nanoparticle (2′) with controlled size at the apex (1′) of the tip (1). Alternatively, the thermal treatment allows locating a group of aggregated nanoparticles (2″) with controlled size at the apex (1′).

[0074] Also, in the context of the present invention we will define the term “cluster (2″)” of one or more nanoparticles (2′) as an aggregate of one or more nanoparticles (2′), in physical contact between the conforming nanoparticles (2′) of the cluster (2″).

[0075] In addition to the above definitions, the expression “physical contact” between nanoparticles (2) and/or clusters (2″) is to be understood as comprising a separation distance (the mean separation between nearest neighbouring nanoparticles (2′)) between the conforming nanoparticles (2′) of the cluster (2″) of less than 0.5 nm.

[0076] On the other hand, the main technical feature of such cluster (2″) is that it is isolated, with “no physical contact” with other nanoparticles (2) of the tip (1). The term “no physical contact” between the cluster (2″) and other nanoparticles (2) is defined as a distance d of at least 0.5 nm, as shown in FIG. 4.

[0077] FIGS. 4a and 4b show a cluster (2″) that is placed at the tip apex (1′) spaced at least at a distance d from any other nanoparticle (2). In FIG. 4a the cluster is formed by one nanoparticle, while in FIG. 4b the cluster is formed by several nanoparticles.

[0078] In a measurement, the afore-defined cluster (2″) interacts with the sample and generates near-field interactions of different nature, depending on the composition of the cluster (2″).

[0079] For example, with clusters of conductive material with plasmonic properties, a highly intensive evanescent field at the apex (1′) is generated. The key to this electromagnetic behaviour is to prevent neighbouring nanoparticles (2) of the tip (1) (not at the apex (1′), but in the surroundings), which affect the plasmon resonance and the optical response of the cluster of nanoparticles (2″). For this reason, it is necessary to have “no physical contact” to isolate material in the tip apex (1′).

[0080] In the case of clusters of materials with magnetic properties, a near-field magnetostatic interaction between the tip (1) and the sample is generated. The key again is to prevent neighbouring nanoparticles (2) of the tip (1) (not at the apex (1′), but in the surroundings), which would lead to a collective behaviour of the magnetic nanoparticles, behaving as a continuous coating. The presence of a cluster of nanoparticles (2″) minimises the stray field and thus avoids perturbations of the magnetic structure of the sample measured. For this reason it is necessary to have “no physical contact” to isolate material in the apex (1″).

[0081] In a preferred embodiment of the invention, the method of production of a system according to any of the claims, comprises the following steps: [0082] a) providing an a tip (1) comprising an apex (1′); [0083] b) depositing a plurality of nanoparticles (2, 2′) with a diameter between 0.5 and 100 nm on the tip (1); [0084] c) applying a thermal treatment on the tip (1) with nanoparticles (2, 2′) deposited in the previous step b) and reaching a temperature between 320 and 1275 K (50 and 1000 Celsius) for the nanoparticles (2, 2′), maintaining such temperature at least until the melting temperature of one or more nanoparticles (2, 2′) is achieved, so that the plurality of nanoparticles (2, 2′) suffers a change in its diameter distribution and nanoparticle density; forming a cluster (2″), wherein said cluster (2″) is spaced from any other nanoparticle (2) of the tip (1) at least a distance d of 0.5 nm; [0085] d) cooling the tip (1) and nanoparticles (2, 2′) down to room temperature.

[0086] In this way, the final size of the nanoparticles (2) can be controlled with the applied temperature. This method obtains nanoparticles (2) with bigger size separated not connected to each other, leaving the bare base material of the tip (1) between them, as shown in FIG. 2c.

[0087] Preferably, the applied temperature will depend on the material and size and density of the nanoparticles (2).

[0088] Preferably, the cluster (2″) of one or more nanoparticles is spaced from any other nanoparticle (2) of the tip (1) a distance d of at least 1 nm. More preferably, the cluster (2″) of one or more nanoparticles is spaced from any other nanoparticle (2) of the tip (1) a distance d of at least 5 nm. More preferably, the cluster (2″) of one or more nanoparticles is spaced from any other nanoparticle (2) of the tip (1) a distance d of at least 10 nm. More preferably, the cluster (2″) of one or more nanoparticles is spaced from any other nanoparticle (2) of the tip (1) a distance d of at least 100 nm. More preferably, the cluster (2″) of one or more nanoparticles is spaced from any other nanoparticle (2) of the tip (1) a distance d of at least 1000 nm.

[0089] Even more preferably, the cluster (2″) of one or more nanoparticles is spaced from any other nanoparticle (2) of the tip (1) an “infinite” distance, meaning that there are no other nanoparticles (2) on the tip (1), apart from the ones at the apex (1′), conforming the cluster (2″).

[0090] In yet another preferred embodiment of the invention, the time of application of the thermal treatment will last between 1 millisecond and 2 hours.

[0091] In yet another preferred embodiment of the invention, the time of the cooling treatment will last between 10 seconds and 2 hours.

[0092] In yet another preferred embodiment of the invention, the thermal treatment in step c) is applied to a cantilever (4) as shown in FIG. 3, where a thermal gradient is obtained, that increases as it gets closer to the apex (1′) of the tip (1).

[0093] The method is suitable for its use in the fabrication of tips for many SPM techniques as: magnetic force microscopy, tip-enhanced Raman spectroscopy, nano infrared microscopy, Kelvin probe force microscopy, piezoresponse force microscopy or scanning capacitance microscopy.

[0094] Another main object of the invention refers to a system obtained through the aforementioned method. Such system comprises a tip (1) and an apex (1′), and a thermally treated coating of nanoparticles (2), with a cluster (2″) of one or more nanoparticles (2′) attached to the apex (1′), in isolation and with no physical contact with the rest of the nanoparticles (2) of the tip (1), as previously defined.