NANOMECHANICAL MASS FLOW METER AND CONTROLLER DEVICE, METHOD, AND ASSOCIATED USES THEREOF

FIELD OF THE INVENTION

The present invention lies within the technical field of microfluidics. More specifically, the invention relates to a nanomechanical mass flow meter and controller device for precise quantitative control of liquid transport in open nanofluidic systems, a method of real-time mass flow measurement and control using said device, and associated uses thereof.

BACKGROUND OF THE INVENTION

The precise manipulation of liquids at the micro and nanoscales is a key issue in several scientific fields, including cell biology, chemistry, engineering, and printing or patterning. Closed systems (i.e., channels, nozzles, or tubes) are mainly used for handling fluids at the submillimeter scale. However, due to their confined geometry, said fluidic systems pose several technical challenges, such as high flow resistance and propensity to clogging and bubble-driven flow perturbation [Dong et al (2013). ACS Nano, 7 (11), 10371-10379; Teh et al (2008). Lab Chip, 8 (2), 198-220; Rossi et al (2004). Nano Lett., 4 (5), 989-993],

Open fluidic systems are consolidating as an alternate technological approach for the detection and manipulation of very small volumes due to lower hydrodynamic resistance, simplicity of fabrication, ease in cleaning, accessibility for liquid handling and increased gas bubbles elimination. Characterized by having, at least, one area of the device open to air, these systems are capable of transporting liquid droplets directionally by gradient in shape, gradient in surface wettability or DC/AC voltage application, enabling self-driven liquid flow with exceptionally large liquid/channel volume ratios. Among them are flexible fiber arrays [Duprat et al (2012). Nature, 482 (7386), 510-513], rigid nanowires [Huang et al (2013). Nat. Nanotechnol., 8 (4), 277-281], spider silks [Ju et al. (2014). Acc. Chem. Res., 47(8), 2342- 2352], cactus spines [Ju et al (2012). Nat. Commun., 3 (1), 1-6], and conical wires [Wang et al (2014). ACS nano, 8(9), 8757-8764],

Despite some degree of qualitative control of liquid transport in open fluidic systems, precise quantitative control is currently limited by the difficulty to provide precise flow measurements. So far, flow measurements are mostly based on the estimation of volume changes of liquid droplets acting as initial reservoirs or being involved in the transport itself, through diverse imaging techniques [Shin, J. H. et al (2019). R. Soc. Open Sci., 6(2), 180657; He, M. et al. (2018). Adv. Funct. Mater., 28(49), 1800187; Hwal Shin, J. et al. (2015). Sci. Rep., 5(1), 1-9; Huang, J. Y. et al (2013). Nat. Nanotechnol., 8 (4), 277-281 ; Li, K. et al (2013). Nat. Commun., 4(1), 1-7], Besides the limitations concerning accuracy, these estimations are useless for the development of compact integrated devices and instruments that fully exploit the functional potential of open fluidic systems.

The present invention proposes a solution to the technical problem mentioned above, by means of a nanomechanical mass flow meter and controller device that allows real-time measurement and control of mass flow in open nanofluidic systems.

BRIEF DESCRIPTION OF THE INVENTION

A first object of the present invention relates to a nanomechanical mass flow meter and controller device for open nanofluidic systems comprising: a mechanical resonator sensor comprising receiving means adapted for receiving a mass of conducting liquid disposed thereon, wherein said mechanical resonator sensor possesses at least one mechanical vibration mode selectable in one or more working frequencies; selecting means adapted for selecting a working frequency corresponding to one mechanical vibration mode of the mechanical resonator sensor; and, monitoring means adapted for monitoring the mechanical spectra of the coupled system conformed by the mass of conducting liquid and the mechanical resonator sensor.

Advantageously, the device of the invention further comprises: a reservoir adapted for containing the mass of conducting liquid; positioning means adapted for adjusting the relative position of the reservoir with respect to the receiving means; and, dispensing means adapted for dispensing the mass of conducting liquid contained in the reservoir on the receiving means, through the application of a voltage between the reservoir and the receiving means.

Within the scope of interpretation of the present invention, the expression “conducting liquid will be understood as any liquid solution that contains ions (i.e. , atoms or small groups of atoms that have a positive or negative electrical charge). Examples of conducting liquids are any type of salt dissolved in water and ionic liquids (that is, molten salts originated from the association of organic cations and organic/inorganic anions).

In a preferred embodiment of the device of the invention, the mechanical resonator sensor comprises a cantilever resonator attached to a nanowire, or a nanowire resonator. In both cases, the nanowire acts as receiving means. Preferably, the nanowire resonator or the nanowire attached to the cantilever resonator comprises a metal or a semiconductor material and, more preferably, silicon (Si), germanium (Ge), silicon carbide (SiC), indium arsenide (InAs), indium phosphide (InP), gallium arsenide (GaAs), gallium phosphide (GaP), indium antimonide (InSb), gallium antimonide (GaSb), gallium nitride (GaN), indium nitride (I nN) , aluminum nitride (AIN), or any combination thereof.

In another preferred embodiment of the device of the invention, the reservoir comprises a support made of a conductive material, preferably, gold or platinum-iridium.

In yet another preferred embodiment of the device of the invention, the reservoir is adapted for containing one or more droplets of an ionic liquid. Preferably, said ionic liquid comprises: 1 ,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide; or 1-butyl-3-methylimidazolium tetrafluoroborate; or diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide.

In yet another preferred embodiment of the device of the invention, the selecting means comprises one or more piezoelectric, optomechanical, electrostatic, capacitive, resistive, or photothermal actuators to excite the flexural modes of the mechanical resonator sensor.

In yet another preferred embodiment of the device of the invention, the monitoring means comprises one or more photodetectors, optomechanical, electrostatic, capacitive, or resistive transducers to detect nanoscale deflections of the mechanical resonator sensor.

In yet another preferred embodiment of the device of the invention, said device further comprises laser emission means arranged in such a way that the emitted laser beam is reflected or backscattered by the mechanical resonator sensor and collected by the monitoring means. In yet another preferred embodiment of the device of the invention, said device further comprises laser focusing means adapted for focusing the laser beam reflected or backscattered by the mechanical resonator sensor on the monitoring means.

In yet another preferred embodiment of the device of the invention, said device further comprises imaging means adapted for taking optical images of the mechanical resonator sensor and the reservoir.

In yet another preferred embodiment of the device of the invention, said device further comprises processing means configured to process an electrical signal from the monitoring means. Preferably, said processing means comprises a digital acquisition (DAQ) board or a lock-in amplifier (LIA).

A second object of the invention relates to a method for controlling mass flow in open nanofluidic systems by means of a device according to any of the embodiments herein described. Advantageously, the method comprises performing the following steps: loading the reservoir with a conducting liquid; positioning, with the positioning means, the conducting liquid loaded into the reservoir in contact with the receiving means; depositing a mass of conducting liquid on the receiving means by applying a voltage between the reservoir and the receiving means with the dispensing means during the period of time At, monitoring, with the monitoring means, the mechanical spectra of the mechanical resonator sensor before and after depositing the mass of conducting liquid on the receiving means; determining the mass of conducting liquid deposited on the receiving means from the mechanical spectra of the mechanical resonator sensor by means of the expression: wherein m _L is the mass of conducting liquid deposited on the receiving means; m _r and ) are the effective mass and mechanical resonance frequency of the mechanical resonator sensor before depositing the mass of conducting liquid on the receiving means, respectively; and Zto is the difference between the mechanical resonance frequencies of the mechanical resonator sensor with and without the deposited mass of conducting liquid on the receiving means. In a preferred embodiment of the method of the invention, said method further comprises a step of determining a mass flow rate of conducting liquid m _L along the receiving means by dividing the mass of conducting liquid m _L by the period of time At.

In another preferred embodiment of the method of the invention, the voltage, in absolute value, applied by the dispensing means is below 4 V. Preferably, between 1 and 4 V and, more preferably, between 2 and 3 V.

In another preferred embodiment of the method of the invention, the conducting liquid loaded in the reservoir comprises an ionic liquid and is subject to a pressure lower than 1 atm.

A third object of the invention relates to a synthesis method of semiconductor nanowires as receiving means of a device according to any of the embodiments herein described, based on vapor-liquid-solid mechanism for single-crystal growth. Advantageously, the method comprises performing the following steps in any technically possible order: a) providing a growth semiconductor substrate and a secondary semiconductor substrate; b) depositing a first mass of a metal catalyst on the growth semiconductor substrate; c) depositing a second mass of the same metal catalyst used in step b) on the secondary semiconductor substrate; d) after steps b) and c), placing growth and secondary substrates into a chemical- vapor-deposition-chamber at a given temperature, forming a plurality of metalsemiconductor alloy droplets; and, e) exposing the metal-semiconductor alloy droplets to a precursor gas flow, undergoing chemical vapor deposition (CVD). In this way, the precursor gas reacts and/or decomposes on the growth semiconductor substrate to produce the semiconductor nanowires.

Under the scope of interpretation of the present invention, the expression “growth semiconductor substrate” will be understood as any semiconductor substrate wherein a plurality of nanowires can be formed by following the method of the invention. Furthermore “metal catalyst” will be understood as any metal material that can act as a seed of the growth of a semiconductor nanowire.

Unlike technological approaches known in the prior art, the synthesis method of the invention allows fabricating semiconductor nanowires up to 50 pm in length with controllable tapered morphology and orientation. Tapered cross-sectional semiconductor nanowires have been ascribed to a gradual decrease of the metal-semiconductor alloy droplets diameter during the nanowire growth process due to loss of metal atoms by incorporation into semiconductor lattice, etching by a byproduct of hydrogen reduction of the precursor gas, and/or vaporization of intermediate compounds dragged by the precursor gas flow. This also implies an eventual limitation of the maximum nanowire length (20-30 pm). The method of the invention mitigates this effect by placing a secondary semiconductor substrate covered with the same metal catalyst as the growth semiconductor substrate into the CVD-chamber. Said secondary semiconductor substrate is positioned with respect to the growth semiconductor substrate in such a way that the precursor gas flow reaches first said secondary substrate. In this way, the loss of metal atoms normally produced at the growth substrate is partially compensated by the arrival of intermediate compounds created at the secondary semiconductor substrate.

In a preferred embodiment of the synthesis method of the invention, step d) is performed at a temperature comprised between 300-900°C, and more preferably between 800-900°C for 5-10 minutes.

In another preferred embodiment of the synthesis method of the invention, the metalsemiconductor alloy droplets are exposed to a precursor gas flow for 10-50 minutes in step e).

In a preferred embodiment of the synthesis method of the invention, the mass of metal catalyst deposited on the secondary semiconductor substrate in step c) comprises a higher concentration than the mass of metal catalyst deposited on the growth semiconductor substrate in step b). Preferably, the concentration of metal catalyst deposited on the secondary semiconductor substrate is between 10-20 times higher than the concentration of metal catalyst deposited on the growth semiconductor substrate.

In another preferred embodiment of the synthesis method of the invention, said method further comprises performing the following steps: purging the chemical-vapor-deposition-chamber with an inert gas and/or stabilizing a continuous precursor gas flow before step e); and/or purging the chemical-vapor-deposition-chamber with an inert gas after step e), to remove precursor gas and intermediate compounds generated during the chemical vapor deposition.

In yet another preferred embodiment of the synthesis method of the invention, the precursor gas comprises silicon tetrachloride (SiCU), a mixture of hydrogen (H ₂ ) and silicon tetrachloride (SiCU), silane (SH4), disilane (Si ₂ H6), dichlorosilane (SiH ₂ CI ₂ ), or any possible combination thereof.

In yet another preferred embodiment of the synthesis method of the invention, wherein the mechanical resonator sensor comprises a nanowire resonator, the growth semiconductor substrate crystal orientation is (111) to obtain nanowires aligned perpendicularly to the growth semiconductor substrate. Alternatively, if the mechanical resonator comprises a cantilever resonator attached to a nanowire, the growth semiconductor substrate crystal orientation is preferably (110) to obtain nanowires aligned with the longitudinal axis of the cantilever resonator, which is preferably oriented along a (111) crystal orientation.

In yet another preferred embodiment of the synthesis method of the invention, the growth and/or secondary semiconductor substrate/s comprise/s: silicon; or a compound from the family of lll-V semiconductors; preferably, InAs, InP, GaAs, GaP, InSb, GaSb; or a compound from the family of Ill-nitrides; preferably, GaN, InN, AIN); or any combination thereof.

In yet another preferred embodiment of the synthesis method of the invention, the metal catalyst comprises Au, Ag, Al, Bi, Cd, Co, Cu, Dy, Fe, Ga, Gd, In, Mg, Mn, Ni, Os, Pb, Pd, Pt, Te, Ti, Zn, or any combination thereof.

In yet another preferred embodiment of the synthesis method of the invention, the step/s of depositing a first and/or a second mass of a metal catalyst on the growth and/or secondary semiconductor substrate/s comprises vaporization, spin coating, or lithography. In this way, the metal catalyst can be deposited over the surface of the growth and/or secondary semiconductor substrate/s forming a layer or as localized metal spots.

A fourth object of the invention relates to the use of a device according to any of the embodiments herein described for controlling mass flow in printing, patterning, proteomic assays, and/or chemical reactions. DESCRIPTION OF THE FIGURES

Figure 1 shows the device of the invention according to one of its preferred embodiments, comprising a cantilever resonator attached to a nanowire as mechanical resonator sensor.

Figure 2 shows the device of the invention according to another of its preferred embodiments, comprising a nanowire resonator as mechanical resonator sensor.

Figure 3 shows the stepwise method for controlling mass flow in open nanofluidic systems by means of the device of the invention in one of its preferred embodiments: (a) monitoring mechanical spectra of the mechanical resonator sensor before depositing a mass of conducting liquid on the receiving means; (b) positioning the receiving means in contact with the conducting liquid loaded into the reservoir and depositing a mass of conducting liquid m _L on the receiving means by applying a voltage between the reservoir and the receiving means with the dispensing means; and (c) monitoring the mechanical spectra of the mechanical resonator sensor after depositing the mass of conducting liquid m _L on the receiving means.

Figure 4a shows the stepwise reduction in the mechanical resonance frequency of the cantilever resonator of the device of the invention in one of its preferred embodiments during the performance of the method of the invention five consecutive times. Figure 4b shows the amount of mass of conducting liquid deposited on the nanowire during each repetition of the method of the invention. Average deposited liquid mass is represented by a dashed line.

Figure 5 shows the amount of liquid mass deposited on the nanowire of the device of the invention in one of its preferred embodiments following the method of the invention as a function of the time that the conducting liquid and the nanowire are in contact and under a voltage. The mechanical resonator of the device of the invention comprises a cantilever resonator attached to a nanowire. A linear fitting of the data is included.

Figure 6 shows the liquid mass flow rate along the nanowire attached to the cantilever resonator of the device of the invention in one of its preferred embodiments depending on the voltage applied between the reservoir and the nanowire with the dispensing means during the performance of the method of the invention. Figure 7a shows the stepwise reduction in the mechanical resonance frequency of the nanowire resonator of the device of the invention in one of its preferred embodiments during the performance of the method of the invention ten consecutive times. Figure 7b shows the amount of mass of conducting liquid deposited on the nanowire resonator during each repetition of the method of the invention. Average deposited liquid mass is represented by a dashed line.

Figure 8 shows the amount of liquid mass deposited on the nanowire resonator of the device of the invention in one of its preferred embodiments following the method of the invention as a function of the time that the conducting liquid and the nanowire resonator are in contact and under a voltage. A linear fitting of the data is included.

Figure 9 shows the liquid mass flow rate along the nanowire resonator of the device of the invention in one of its preferred embodiments depending on the voltage applied between the conducting liquid and the nanowire resonator with the dispensing means during the performance of the method of the invention.

NUMERICAL REFERENCES USED IN THE DRAWINGS

In order to provide a better understanding of the technical features of the invention, the referred Figures 1-9 are accompanied of a series of numerical references which, with an illustrative and non-limiting character, are hereby represented:

DETAILED DESCRIPTION OF THE INVENTION

While aspects of the subject matter of the present disclosure may be embodied in a variety of forms, the following description and accompanying drawings are merely intended to disclose some of these forms as specific examples of the subject matter. Accordingly, the subject matter of this disclosure is not intended to be limited to the forms or embodiments so described and illustrated.

Unless defined otherwise, all terms of art, notations and other technical terms or terminology used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs.

As described in preceding paragraphs, one object of the present invention relates to a nanomechanical mass flow meter and controller device for precise quantitative control of liquid transport in open nanofluidic systems. To do so, said device comprises: a mechanical resonator sensor (1) comprising receiving means (2) adapted for receiving a mass of conducting liquid (5’) disposed thereon, wherein said mechanical resonator sensor (1) possesses at least one mechanical vibration mode selectable in one or more working frequencies; selecting means (3) adapted for selecting a working frequency corresponding to one mechanical vibration mode of the mechanical resonator sensor (1); and monitoring means (4) adapted for monitoring the mechanical spectra of the coupled system conformed by the mass of conducting liquid (5’) and the mechanical resonator sensor (1).

Advantageously, the device of the invention further comprises: a reservoir (5) adapted for containing the mass of conducting liquid (5’); positioning means (6) adapted for adjusting the relative position of the reservoir (5) with respect to the receiving means (2); and, dispensing means (7) adapted for dispensing the mass of conducting liquid (5’) contained in the reservoir (5) on the receiving means (2), through the application of a voltage between the reservoir (5) and the receiving means (2). As previously described, the expression “conducting liquid” will be herein understood as any liquid solution that contains ions, including any type of salt dissolved in water and ionic liquids.

In different embodiments of the device of the invention, the mechanical resonator sensor (1) can comprise, for instance, a cantilever resonator attached to a nanowire (see Figure 1), or a nanowire resonator (see Figure 2). In both cases, the nanowire acts as receiving means (2) and typically comprises a metal or semiconductor material and, more preferably, silicon (Si), germanium (Ge), silicon carbide (SiC), indium arsenide (InAs), indium phosphide (InP), gallium arsenide (GaAs), gallium phosphide (GaP), indium antimonide (InSb), gallium antimonide (GaSb), gallium nitride (GaN), indium nitride (InN), aluminum nitride (AIN), or any combination thereof.

The selecting means (3) comprises a piezoelectric actuator to excite the flexural modes of the mechanical resonator sensor (1). However, other actuation systems can also be employed in the context of the invention, such as optomechanical, electrostatic, capacitive, resistive or photothermal actuators.

The monitoring means (4) comprises a photodetector to detect nanoscale deflection of the mechanical resonator sensor (1). To do that, laser emission means (8), for example a fiber- coupled diode laser, emits a laser beam in such a way that said beam is reflected or backscattered by the mechanical resonator sensor (1) and focused on the photodetector (4) by laser focusing means (9), typically, a 10X objective. Said photodetector (4) can comprise a photodiode of two or four closely jointed segments, or an unsegmented photoreceiver coupled to a low noise transimpedance amplifier. However, other types of transduction systems can also be employed in the context of the invention, such as optomechanical, electrostatic, capacitive, or resistive transducers.

In a preferred embodiment of the device of the invention (see Figure 2), the power of the laser emitted by the laser emission means (8) is controlled with a variable attenuator (8’), preferably in the range from 3.6 pW to 14.4 pW, high enough to resolve the thermomechanical signal of nanowires at acquisition times equal or lower than 1 second but without inducing any observable optomechanical back-action effect. The polarization of said emitted laser beam is aligned with the longitudinal axis of the mechanical resonator sensor (1) by means of birefringence loops (8”), so that the backscattered intensity is maximize and, hence, transduction sensitivity. After this fiber stage, a triplet lens collimator (8”’) provides a nearly Gaussian free-space laser beam, with its optical axis oriented perpendicular to the longitudinal axis of the mechanical resonator sensor (1). Collimated laser beam is focused on the mechanical resonator sensor (1) using a 10X objective (9) with 0.28 numerical aperture, which results into a laser spot waist diameter of around 6 pm.

The reservoir (5) comprises a conductive support, preferably made of gold or platinumiridium, and adapted for preferably containing one or more droplets of any of the following ionic liquids: 1 ,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3- methylimidazolium tetrafluoroborate, or diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide. The relative position of said reservoir (5) with respect to the receiving means (2) is controlled by an XYZ nanopositioning stage (6).

Imaging means (10), preferably a CCD camera with white light illumination, is coupled to the mechanical resonator sensor (1) to follow in real-time the approximation of the reservoir (5) to the receiving means (2) and, optionally, the deposition of a mass of conducting liquid (5’) on said means (2).

The resulting electrical signal from the photodetector (4) is then processed by processing means (11), either a digital acquisition (DAQ) board or a lock-in amplifier (LIA). LIA reference signal is delivered to the selecting means (3) in case of driven vibrations. The DAQ board is synchronized both with a waveform generator (WFG), which can also be connected to the selecting means (3), and with positioning means (6), allowing the acquisition of the signal of the photodetector as the relative mechanical resonator sensor (1)-laser beam position is scanned. Measurements can be performed in diverse environmental conditions, including, but not limited to, high vacuum (~10 ^-5 mbar), and at substrate temperatures of around 300 K.

A second object of the invention relates to a method for controlling mass flow in open nanofluidic systems by means of a device according to any of the embodiments herein described. Advantageously, the method comprises performing the following steps (see Figure 3): loading the reservoir (5) with a conducting liquid (5’); positioning, with the positioning means (6), the conducting liquid (5’) loaded into the reservoir (5) in contact with the receiving means (2) during a period of time At, depositing a mass of conducting liquid (5’) on the receiving means (2) by applying a voltage between the reservoir (5) and the receiving means (2) with the dispensing means (7) during the period of time At monitoring, with the monitoring means (4), the mechanical spectra of the mechanical resonator sensor (1) before and after depositing the mass of conducting liquid (5’) on the receiving means (2); determining the mass of conducting liquid (5’) deposited on the receiving means (2) from the mechanical spectra of the mechanical resonator sensor (1) by means of the expression: wherein m _L is the mass of conducting liquid (5’) deposited on the receiving means (2); m _r and M are the effective mass and mechanical resonance frequency of the mechanical resonator sensor (1) before depositing the mass of conducting liquid (5’) on the receiving means (2), respectively; and Zto is the difference between the mechanical resonance frequencies of the mechanical resonator sensor (1) with and without the deposited mass of conducting liquid (5’) on the receiving means (2).

Additionally, said method can further comprises a step of determining a mass flow rate of conducting liquid, m _L , along the receiving means (2) by dividing the mass of conducting liquid (5’), m _L , by the period of time At.

In a preferred embodiment of the method of the invention, the voltage, in absolute value, applied by the dispensing means (7) is below 4 V (both polarities can produce controllable flow). Preferably, between 1 and 4 V and, more preferably, between 2 and 3 V, both in positive and negative values. Under these voltages, conducting liquid (5’) from the reservoir (5) can be made to flow steadily along the receiving means (2) with both great degree of control and high flow rate measurement sensitivity (nanowire resonator: 1-100 ag/s, in vacuum; cantilever resonator attached to a nanowire: 10-100 fg/s, at standard temperature and pressure). Below 1-2 V, depending on the characteristics of the nanowire and the conducting liquid, liquid transport to the nanowires does not occur, and above 3-4 V, chemical reactions result in nanowire etching, preventing accurate measurement and control of the mass flow rate.

In yet another preferred embodiment of the method of the invention, the conducting liquid (5’) loaded in the reservoir (5) comprises an ionic liquid and is subject to a pressure lower than 1 atm. More preferably, the pressure is maintained below 1 atm during the performance of the method of the invention, for example, by introducing the device inside a vacuum chamber. Under these conditions, the ionic liquid (5’) does not evaporate, allowing not only controlling the mass flow of the ionic liquid (5’) in an open nanofluidic system, but also measuring analytes within that liquid (5’) without degrading them (e.g., proteins).

A third object of the invention relates to a synthesis method of semiconductor nanowires as receiving means (2) of a nanomechanical mass flow meter and controller device according to any of the embodiments herein described, based on vapor-liquid-solid mechanism for single-crystal growth. Advantageously, the method comprises performing the following steps in any technically possible order: a) providing a growth semiconductor substrate and a semiconductor secondary substrate; b) depositing a first mass of a metal catalyst on the growth semiconductor substrate; c) depositing a second mass of the same metal catalyst used in step b) on the secondary semiconductor substrate; d) after steps b) and c), placing growth and secondary substrates into a chemical- vapor-deposition-chamber at a given temperature, forming a plurality of metalsemiconductor alloy droplets; and, e) exposing the metal-semiconductor alloy droplets to a precursor gas flow, undergoing chemical vapor deposition (CVD). In this way, the precursor gas reacts and/or decomposes on the growth semiconductor substrate to produce the semiconductor nanowires.

In a preferred embodiment of the synthesis method of the invention, step d) is performed at a temperature comprised between 300-900°C, and more preferably between 800-900°C for 5-10 minutes.

In another preferred embodiment of the synthesis method of the invention, the metalsemiconductor alloy droplets are exposed to a precursor gas flow for 10-50 minutes in step e).

In yet another preferred embodiment of the invention, the precursor gas comprises silicon tetrachloride (SiCU), a mixture of hydrogen (H2) and silicon tetrachloride (SiCU), silane (SH4), disilane (Si2He), dichlorosilane (SiFWh), or any possible combination thereof.

Previously to step e), the chemical-vapor deposition-chamber is usually purged with an inert gas, and the precursor gas flow is stabilized. After step e), said chamber is usually purged again with an inert gas, not only to remove traces of the precursor gas, but also intermediate compounds generated during the chemical vapor deposition.

Steps b) and/or c) can be carried out by vaporization, spin coating, or lithography. In this way, the metal catalyst can be deposited over the surface of the growth and/or secondary semiconductor substrates forming a layer or as localized metal spots.

Unlike technological approaches known in the prior art, the method of the invention allows fabricating semiconductor nanowires up to 50 pm in length with controllable tapered morphology and orientation. Tapered cross-sectional semiconductor nanowires have been ascribed to a gradual decrease of the metal-semiconductor alloy droplets diameter during the nanowire growth process due to metal incorporation into semiconductor lattice, etching by a byproduct of hydrogen reduction of the gaseous silicon precursor, and/or metal vaporization dragged by the precursor gas flow. This also implies an eventual limitation of the maximum nanowire length (20-30 pm). The method of the invention mitigates this effect by placing a secondary semiconductor substrate covered with the same metal catalyst as the growth semiconductor substrate into the CVD-chamber. Said secondary semiconductor substrate is positioned with respect to the growth semiconductor substrate in such a way that the precursor gas flow reaches first said secondary substrate. In this way, the loss of metal atoms normally produced at the growth substrate is partially compensated by the arrival of intermediate compounds created at the secondary semiconductor substrate.

In a preferred embodiment of the invention, the mass of metal catalyst deposited on the secondary semiconductor substrate in step c) comprises a higher concentration than the mass of metal catalyst deposited on the growth semiconductor substrate in step b). Preferably, the concentration of metal catalyst deposited on the secondary semiconductor substrate is between 10-20 times higher than the concentration of metal catalyst deposited on the growth semiconductor substrate.

Preferably, the growth semiconductor substrate crystal orientation is:

(111) for a mechanical resonator sensor (1) comprising a nanowire resonator (1 ,2), to obtain nanowires (2) aligned perpendicularly to the growth semiconductor substrate; or

(110) for a mechanical resonator sensor (1) comprising a cantilever resonator attached to a nanowire (2), to obtain nanowires (2) aligned with the longitudinal axis of the cantilever resonator (1), which is preferably oriented along a (111) crystal orientation. This alignment facilitates that the cantilever resonator, the nanowire (2), and the reservoir (5) are in the same focal plane and can be recorded by the imaging means (10) at the same time.

The growth and/or secondary semiconductor substrate/s preferably comprise/s: silicon; or a compound from the family of lll-V semiconductors; preferably, InAs, InP, GaAs, GaP, InSb, GaSb; or a compound from the family of Ill-nitrides; preferably, GaN, InN, AIN); or any combination thereof.

In yet another preferred embodiment of the invention, the mass of metal catalyst deposited on the growth and/or the secondary semiconductor substrate/s in step b) and/or c) comprises Au, Ag, Al, Bi, Cd, Co, Cu, Dy, Fe, Ga, Gd, In, Mg, Mn, Ni, Os, Pb, Pd, Pt, Te, Ti, Zn, or any combination thereof.

A fourth object of the invention relates to the use of a device according to any of the embodiments herein described for controlling mass flow in printing, patterning, proteomic assays, and/or chemical reactions.

Examples of realization

Example 1 : Depositing a mass of liquid in an open nanofluidic system by means of a nanomechanical mass flow meter and controller device comprising a cantilever resonator attached to a nanowire.

This first example shows the process of depositing a mass of liquid in an open nanofluidic system by means of the device of the invention in one of its preferred embodiments following the method of the invention. Specifically, the mechanical resonator sensor (1) comprises a cantilever resonator m _r = 36 ng) attached to a nanowire, said nanowire acting as receiving means (2). The conducting liquid (5’) loaded into the reservoir (5) is 1 ,2-dimethyl-3- propylimidazolium bis(trifluoromethylsulfonyl)imide. Said process was carried out in air at 300 K.

As shown in Figure 4a, the process starts with the cantilever resonator under a mechanical resonance frequency (w) of 10965 Hz. After putting the conducting liquid (5’), loaded into the reservoir (5), in contact with the nanowire (2) by an XYZ nanopositioning stage (6), a frequency shift (Zto) is caused since a mass of said conducting liquid (5’) is deposited on the nanowire (2) by applying a voltage of 2.7 V between said reservoir (5) and the nanowire (2) for 60 seconds (At) with the dispensing means (7) (see Fig. 4b). To demonstrate the reproducibility of the method of the invention, this process was repeated 4 more times, represented by the stepwise reduction in the mechanical resonance frequency of the cantilever resonator in Figure 4a. The amount of mass of conducting liquid deposited on the nanowire (2) during each repetition of the method of the invention is shown in Figure 4b (mean ± standard deviation: m _L = 3.4 ± 0.6 pg).

Example 2: Liquid transport in an open nanofluidic system by means of a nanomechanical mass flow meter and controller device comprising a cantilever resonator attached to a nanowire at a constant voltage.

This second example shows the process of controlling liquid transport in an open nanofluidic system by means of the device of the invention in one of its preferred embodiments following the method of the invention. Specifically, the mechanical resonator sensor (1) comprises a cantilever resonator m _r = 36 ng) attached to a nanowire, said nanowire acting as receiving means (2). The conducting liquid (5’) loaded into the reservoir (5) is 1 ,2-dimethyl-3- propylimidazolium bis(trifluoromethylsulfonyl)imide. Said process was carried out in air at 300 K.

The process starts with the cantilever resonator (1) under a mechanical resonance frequency of 10965 Hz. After putting the conducting liquid (5’), loaded into the reservoir (5), in contact with the nanowire (2) by an XYZ nanopositioning stage (6), a frequency shift (Zto) is caused since a mass of said conducting liquid (5’) is deposited on the nanowire (2) by applying a constant voltage of 2.7 V between the reservoir (5) and the nanowire (2) with the dispensing means (7). The amount of mass of conducting liquid (5’) deposited in the nanowire (2) as a function of the time that the conducting liquid (5’) and the nanowire (2) are in contact and under voltage is shown in Figure 5, obtaining a controlled mass flow rate of conducting liquid (5’) m _L of 60 fg/s.

Example 3: Liquid transport in an open nanofluidic system by means of a nanomechanical mass flow meter and controller device comprising a cantilever resonator attached to a nanowire depending on applied voltage. This third example shows liquid transport in an open nanofluidic system by means of the device of the invention in one of its preferred embodiments depending on the voltage applied by the dispensing means (7) between the reservoir (5) and the receiving means (2) of the mechanical resonator sensor (1). Specifically, the mechanical resonator sensor (1) comprises a cantilever resonator (m _r = 17 ng) attached to a nanowire, said nanowire acting as receiving means (2). The conducting liquid (5’) loaded into the reservoir (5) is 1 ,2- dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide. Said process was carried out in air at 300 K.

The process starts with the cantilever resonator under a mechanical resonance frequency (w) of 6170 Hz. After putting the conducting liquid (5’), loaded into the reservoir (5), in contact with the nanowire (2) by an XYZ nanopositioning stage (6), an increasing voltage between the reservoir (5) and the nanowire (2) is applied by the dispensing means (7). As shown in Figure 6, liquid transport to the nanowire (2) does not occur below 2 V, and above 3 V approx., chemical reactions result in nanowire etching, preventing accurate measurement and control of the conducting liquid mass flow rate.

Example 4: Depositing a mass of liquid in an open nanofluidic system by means of a nanomechanical mass flow meter and controller device comprising a nanowire resonator.

This forth example shows the process of depositing a mass of liquid in an open nanofluidic system by means of the device of the invention in one of its preferred embodiments following the method of the invention. Specifically, the mechanical resonator sensor (1) comprises a nanowire resonator m _r =722 fg), wherein a controlled mass of conducting liquid (5’) will be deposited. The conducting liquid (5’) loaded into the reservoir (5) is 1 ,2-dimethyl-3- propylimidazolium bis(trifluoromethylsulfonyl)imide. Said process was carried out in in high vacuum (10 ^-5 mbar) at a temperature of 300 K.

As shown in Figure 7a, the process starts with the nanowire resonator under a mechanical resonance frequency (w) of 245597 Hz. After putting the conducting liquid (5’), loaded into the reservoir (5), in contact with the nanowire resonator by an XYZ nanopositioning stage (6), a frequency shift (Zto) is caused since a mass of said conducting liquid (5’) is deposited on the nanowire resonator by applying a voltage of 1 .4 V between the reservoir (5) and the nanowire resonator for 5 seconds (At) with the dispensing means (7) (see Fig. 7b). To demonstrate the reproducibility of the method of the invention, this process was repeated 9 more times, represented by the stepwise reduction in the mechanical resonance frequency of the nanowire resonator in Figure 7a. The amount of mass of conducting liquid (5’) deposited on the nanowire resonator during each repetition of the method of the invention is shown in Figure 7b (mean ± standard deviation: m _L = 29 ± 7 ag).

Example 5: Liquid transport in an open nanofluidic system by means of a nanomechanical mass flow meter and controller device comprising a nanowire resonator at a constant voltage.

This fifth example shows the process of controlling liquid transport in an open nanofluidic system by means of the device of the invention in one of its preferred embodiments following the method of the invention. Specifically, the mechanical resonator sensor (1) comprises a nanowire resonator m _r = 727 fg), wherein a controlled mass of conducting liquid (5’) will be deposited. The conducting liquid (5’) loaded into the reservoir (5) is diethylmethyl(2- methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide. Said process was carried out in in high vacuum (10' ⁵ mbar) at a temperature of 300 K.

The process starts with the nanowire resonator under a mechanical resonance frequency (w) of 237342 Hz. After putting the conducting liquid (5’), loaded into the reservoir (5), in contact with the nanowire resonator by an XYZ nanopositioning stage (6), a frequency shift (Zlw) is caused since a mass of said conducting liquid (5’) is deposited on the nanowire resonator by applying a voltage of 1.4 V between the reservoir (5) and the nanowire resonator with the dispensing means (7). The amount of liquid mass deposited in the nanowire resonator as a function of the time that the conducting liquid (5’) and the nanowire resonator are in contact and under voltage is shown in Figure 8, obtaining a controlled mass flow rate of conducting liquid m _L of 4.3 ag/s. means of a nanomechanical mass flow meter and controller device a nanowire resonator depending on

This sixth example shows liquid transport in an open nanofluidic system by means of the device of the invention in one of its preferred embodiments depending on the voltage applied by the dispensing means (7) between the reservoir (5) and the receiving means (2) of the mechanical resonator sensor (1). Specifically, the mechanical resonator sensor (1) comprises a nanowire resonator (m _r = 621 fg), wherein a controlled mass of conducting liquid (5’) will be deposited. The conducting liquid (5’) loaded into the reservoir (5) is diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide. Said process was carried out in in high vacuum (10' ⁵ mbar) at a temperature of 300 K.

The process starts with the nanowire resonator under a mechanical resonance frequency (w) of 228828 Hz. After putting the conducting liquid (5’), loaded into the reservoir (5), in contact with the nanowire resonator by an XYZ nanopositioning stage (6), an increasing voltage between the reservoir (5) and the nanowire resonator is applied by the dispensing means (7). As shown in Figure 9, liquid transport to the nanowire resonator does not occur below 1.4 V, and above 3 V approx., chemical reactions result in nanowire etching, preventing accurate measurement and control of the conducting liquid mass flow rate.